Birmingham goalkeeper Ben Foster, who made a string of key saves in the League Cup final against Arsenal, cleared counterpart Wojciech Szczesny of blame for the blunder that led to the late winner.
Szczesny and Koscielny both went for the ball from Nikola Zigic's glancing header from a long pass into the box. The ball landed at the feet of Birmingham striker Obafemi Martins, who slid it into the net in the 89th minute to give his team a 2-1 win on Sunday at Wembley Stadium.
Foster told The Associated Press that he believes Koscielny was at fault and that Szczesny had probably called to take the ball.
''The center half kind of just got involved when he didn't really need to,'' Foster said. ''I thought the goalkeeper probably shouted for it. Obviously, there's a lot of noise in Wembley. He (Koscielny) couldn't hear and he's just tried to clear when it was obvious that it would drop into his (the goalkeeper's) hands.
''It's one of those things: when it's not heard, it kind of looks bad on the goalkeeper. But if the defender had just left it, it would have dropped right into his (Szczesny's) hands.''
Foster said it's very difficult for goalkeepers and defender to prepare for an incident like that because it's virtually impossible to recreate in training.
''It's just one of those things that you hope it really doesn't happen to you,'' he said.
Foster doesn't think the Polish 'keeper should have done something rash like barging into Koscielny to get him out of the way, because ''you're going into the unknown and you really don't know what's going to happen.
Szczesny, who turns 21 in April, recently became Arsenal's first-choice keeper after injuries to Lukasz Fabianski and Manuel Almunia. He has made a strong impression on Arsenal manager Arsene Wenger and has mostly put in solid performances.
It has been a long road for the 'keeper, who broke both arms in a freak accident while lifting weights in the gym at Arsenal's training ground in 2008.
Foster had some kind words for Szczesny, calling the Pole a good, young prospect for Arsenal.
''He needs to move on from this and put it behind him and carry on with the good form he is showing already,'' Foster said.
Foster, who won the League Cup with Manchester United in a penalty shootout in 2009, made several important saves throughout the match. He denied Andrei Arshavin with his legs in the opening minutes and thwarted Samir Nasri twice and Nicklas Bendtner toward the end of the match.
