Arsène Wenger, the Arsenal manager, has issued a strongly worded reminder to his young players about the need to show professionalism in every match and expressed concern over his squad's mental strength. Arsenal can seize control of Champions League group G by beating Porto at the Emirates Stadium this evening, but it will take more than that to erase memories of Saturday's shock home defeat by Hull City.

Wenger is seething at what he considers to be one of the worst results in his 12 years at the club and felt sufficiently frustrated yesterday to question his side's mentality for the first time. The Frenchman accused his players of being overconfident against lesser opponents and of assuming that some games would be easy.

“I think we're solid, more solid than ever when we're completely switched on,” Wenger said. “But at the moment, I'm not sure we can focus all the time in every single game, especially when we think the game will be easy. It is a good learning curve because you cannot get worse than Saturday. In the games in which we're favourites, we have to be as difficult to play against as the others.

“When you are young, you think things are going to be easy. When you have a bang on the head like this, you have to understand the need to be consistent. It was a big bang and shows we're not completely there on the mental side. In the past, we have been beaten physically, we have been intimidated. We know we can deal with that now, but we're sometimes a bit too confident in the home games when we need to put some effort in.”

Wenger admitted that the Hull defeat is the surprise result of the Barclays Premier League season, but will resist making wholesale changes against Porto. Samir Nasri returns to the squad after a month out because of a knee injury and Nicklas Bendtner could step up from the bench to partner Emmanuel Adebayor up front. “It is not easy to deal with defeat,” Wenger said. “It does not become easier, as it is the biggest shock of the season. I can understand that for a non-Arsenal fan it was a great day, but for us it was a shocking defeat.

“I was deeply disappointed because I wanted desperately to win. We had a great opportunity to be top of the league if we beat Hull at home.”

Manchester United could become victims themselves of a huge upset this evening when they face Aalborg. Bruce Rioch, the Danish club's coach who preceded Wenger at Arsenal, said yesterday that any positive result against United in their group E match would be like an earthquake, with the visiting team hoping to avoid a repeat of their 1-0 defeat by Copenhagen in this competition two years ago.