John Terry had regained full consciousness by Sunday evening and, although still groggy, could be seen signing autographs and posing happily with the Carling Cup at Cardiff airport. This does not mean, however, that we should stop worrying about him.

It is one thing to play in the manner of Captain Indestructible and quite another to believe that Captain Indestructible genuinely exists. Now we know the images of Terry lying apparently half-dead on the Millennium Stadium turf were snapshots of a moment in time and did not tell the full story, it would be easy to ignore the alarm bells, applaud the courage of this remarkable footballer and move on. Yet to delight in the happy ending misses a wider area of concern.

Terry left hospital after two hours having been admitted with head trauma. Complications from such injuries can emerge anything up to 24 hours after the event. After scans revealed no damage, Terry is believed to have played no small part in his hasty return to the scene of Chelsea’s celebrations (although the club insist he did not discharge himself, as has been suggested, and would not have allowed him to disobey doctors’ orders had he been required to stay in for observation).

Even so, Terry’s iron man act is increasingly troubling. On the dial of good intentions, heroism and stupidity sit side by side. There must be anxiety that Terry is becoming bewitched with leading by physical example and to hell with the consequences, the fate that befell another captain of club and country, Tony Adams.

Terry did not come round until he was in the ambulance on the way to University Hospital, Cardiff, at which point he expressed a desire to redirect the vehicle back to the stadium. This rashness would surely have been the consequence of confusion caused by the knockout, but it also gives a clue to Terry’s instincts, which invariably place the needs of the team above his own.

On April 29 last year, early in the match against Manchester United that brought a second consecutive Premiership title to Stamford Bridge, Terry sustained an ankle injury requiring ten stitches after a tackle by Wayne Rooney. Understandably, he gritted his teeth and played on, with Chelsea protecting a 1-0 lead. In the 61st minute, Joe Cole put Chelsea 2-0 ahead. Terry continued.

In the 73rd minute, Ricardo Carvalho made it 3-0. Terry remained on the pitch. The heroism-stupidity line was crossed after the second goal, particularly in a game in which Chelsea needed only a draw to be crowned champions. It was obvious that Terry played that day in considerable pain and also that, after Cole’s goal, he did not need to. The last team to score three away from home in a league match at Stamford Bridge were West Ham United on September 28, 2002.

Terry’s injury was superficial in comparison with the broken metatarsal that scuppered Rooney and England’s World Cup, but even so he left the ground on crutches that night, having played at least 30 minutes when common sense should have sent him to the bench.

Part of the captain’s role is to show good judgment. Gary Neville failed this test when marching after Lille’s players in Lens last week, provoking a furious reaction from Sir Alex Ferguson. In attempting to leave the field, even with the best intentions, Neville risked making Manchester United part of Uefa’s problem. Ferguson’s response was to leave him out of the game against Fulham, a reminder that a captain’s greatest asset is his intellect.

Terry is a thinker, too. He has to be to carry out the instructions and practices of his club manager, José Mourinho, but he is also a captain who inspires with his courage. English football loves this type, which is why Bryan Robson is revered by all players, managers and supporters of a certain age; yet it is dangerously habit-forming to be the focus of this adoration. The title of Adams’s autobiography, Addicted, does not refer only to his relationship with alcohol.

Adams was another who played when he should not, whose determination to make it as a professional footballer and, once there, to set an impossibly high standard as Arsenal and England captain took a heavy toll on his health. Roy Keane’s drive to be Ferguson incarnate on the field for United brought with it an almost psychotic intensity that in the end proved his downfall at Old Trafford.

Yes, these men will admit that much of the pressure came from within and there is no suggestion that Terry is pursued by the same demons as Adams. Yet his behaviour this season displays a familiar readiness to put his body on the line for his club in a way that can be harmful, long term. The psychological pressures come later, usually as a result of this.

It could be argued that Mourinho could do more to resist his captain’s desire to play at all costs, but there is little doubt that it was the player who declared himself ready to start against Arsenal, just four days after sustaining an ankle injury in Oporto. In the circumstances, at such a crucial stage of the season and with central defenders thin on the ground, perhaps it is no surprise that Mourinho frequently chooses to take the positives from Terry’s heightened sense of duty. Plainly, on Sunday, he was not right. Patched up after his ankle ligament injury, his first-half performance when Arsenal were dominating was hesitant and Carvalho clearly took the defensive lead. Looked at from this perspective, even the fateful collision with Abou Diaby could have been the result of a drop in his usual sharpness.

Terry’s potency in the penalty area stems from two attributes: his remarkable bravery and his quick reading of a situation. He spots a goalscoring opportunity with surprising clarity for a defender and is prepared to put his head where others will not to score it. Terry is no Cristiano Ronaldo, but he is fast over a short distance and sheer determination gets him to the crucial position when opponents are only beginning to wake up to the danger.

By then, in that split second, the ball is in the net, his head is clear of risk and if he takes a kick it might be to his torso; bruising, sure, but nothing serious. On Sunday, he appeared to be slower arriving and by the time he had launched himself at the ball, Diaby had spotted what was unfolding. Terry tried to react with urgency but he was not able to get there in his usual way. His head became synchronised with Diaby’s boot. The result was potential catastrophe.

Terry had a back operation in December but some believe he had been carrying a handicap for longer. Certainly, Terry Venables, the England assistant coach, is on record saying he felt Terry’s movement was hampered for some time, although the player shrugged off all inquiries about his fitness. This is all part of the man-of-steel legend. Adams had a similar attitude during Euro 96, when he played in every game despite hardly being able to train.

And, of course, brave leaders are important. Mourinho sincerely describes Terry as the best defender in the world, George Graham confessed that he frequently thanked God for Adams during matches and when Dave Mackay broke a leg for the second time in a year attempting a comeback in Tottenham Hotspur’s reserves, Bill Nicholson, the manager, cried on hearing the news.

After Terry went down injured on Sunday, it was noticeable that Steve McClaren, the England head coach, left his seat immediately to find out more, despite his next match coming on March 24, four weeks away.

So there are demands and expectations and captains are types who feel highly motivated by the team ethic. Terry plays because he does not want to let his side down and he hopes they will follow his lead. He has been that way from the start. I recall meeting him for the first time on a plane journey to Tel Aviv for a Uefa Cup tie with Chelsea in October 2001, a trip that six of his experienced teammates declined to make. Terry was 20, with clear leadership potential.

Yet Sunday’s events went a step beyond the previous appearances of Captain Fantastic. The game was over when Terry dashed back from hospital. The job had been done. All that remained except to say thank you to Gary Lewin, the Arsenal physiotherapist, who cleared his windpipe and saved his life — and that could have been achieved with a telephone call — were the celebrations, and if Terry’s prime motivation was that he did not wish to miss the fun, then he has more in common with Adams than we imagine.

This is how the chain of events begins. The seemingly indestructible player, the mainstay of his team, the roar of approval as he gives more and more, building a need to be involved, pressure to remain at the centre of the action, to achieve, to risk, all mounting up until a seemingly invincible human being just breaks.

What Terry did on the field in Cardiff was valiant and what he did next was foolish, and with such a thin line between the two he needs to learn to make the distinction. Because in the pressure-cooker of modern football, a man cannot always rely on others to take the right decision for him. Even a superhero has to leave the cape at home sometimes.