It didn’t take a degree in political science from Stanford University to work out what Sam Allardyce was going to do at half-time with his side a goal down, but Ryan Nelsen has such a thing and was able to take full advantage by setting up the equalizer and then scoring one himself as Blackburn ran out easy winners with three second-half goals.

Allardyce pulled off his two wingers, El Hadji Diouf and Morten Gamst Pedersen, at half time, bringing on two centre-forwards, Benni McCarthy and Jason Roberts, and as the starting gun went for the second half about six Blackburn forwards charged forward from the halfway line. Having looked comfortable in the first half and taken the lead through a goal from Jamie O’Hara, Portsmouth duly collapsed.

“I don’t normally get so radical with the substitutions, but it was the worst first-half performance of the season and the fans were right to show their displeasure,” Allardyce said. “We had to do something. It worked tremendously well and the substitutes have created and scored the goals.”

The collapse in the second half hurt Portsmouth manager Paul Hart as he looked to continue a recent run of form that would have moved his side off bottom place and possibly even out of the relegation zone.

O’Hara sounded an early warning with a shot when given acres of space outside the box. By the 16th minute, he had his sights set properly and, unchallenged, produced a second effort that skimmed along the ground from about 30 yards and squeezed inside Paul Robinson’s left-hand post.

Blackburn might have been in further trouble had referee Andre Marriner applied the letter of the law and sent off Pascal Chimbonda after he lashed out with a closed fist into the face of O’Hara, but O’Hara did his former Spurs teammate a big favour by staying on his feet and only a yellow card was brandished.

The home side didn’t need any more luck after that, just the arrival of the light brigade at half-time, which was the cue for David Dunn to start working his tricks from dead-ball situations. Firstly he forced Frederic Piquionne into a desperate clearance which resulted in a corner. Dunn’s corner was planted on the head of Nelsen, whose powerful downward header landed at the feet of Roberts, who was able to spin and score from three yards out.

From Dunn’s inswinging corner from the other side, Nelsen again rose highest and thundered in a header which gave Blackburn the lead and begged serious questions of the marking.

The second-half rout was completed when McCarthy crossed for Roberts to give the forward a second tap-in, this time at the near post. “If you don’t defend set-pieces against Blackburn you are going to be in trouble and that is what happened,” Hart said. “We managed to deal with them in the first half and were in no trouble. I don’t think we were done by genius. We were done by set-pieces.”

Star man: David Dunn (Blackburn) Yellow cards: Blackburn: Nelsen, Givet, Chimbonda Portsmouth: Brown, Boateng, O’Hara

Referee: A Marriner Attendance: 23,110

BLACKBURN: Robinson 5, Chimbonda 5, Samba 7, Nelsen 8, Givet 6, Diouf 4 (McCarthy 56min, 6), Nzonzi 6, Emerton 6, Pedersen 5 (Roberts 46min, 6), Dunn 8, Di Santo 6 (Andrews 84min)

PORTSMOUTH:James 6, Borre 5, Kaboul 6, Wilson 6, Ben Haim 5, Brown 5, Mokoena 6 (Kanu 78min), Boateng 7, O’Hara 7, Dindane 6 (Smith 50min, 6), Piquionne 6