Caretaker boss Roberto Di Matteo admitted Chelsea suffered a hangover from Wednesday night's Champions League win over Benfica after they struggled to beat Wigan.
Branislav Ivanovic put the Blues in front with a strike that was scored from a blatantly offside position, but Wigan were heading for a well-deserved 1-1 draw thanks to substitute Mohamed's Diame's 82nd-minute equaliser.
However, another controversial goal - this time from Juan Mata in the third minute of added-on time - saw Chelsea claim the spoils.
Despite making seven changes from the side that progressed to the Champions League semi-finals in midweek, Di Matteo said: "After you play a midweek game in the Champions League, there's always a concern that you might have a little hangover for the weekend game.
"And even though I tried to make a number of changes for today's game to energise the team, I think the first half, we didn't really get into the game. The tempo was slow and the intensity was low. There's always that concern when you play midweek."
Di Matteo claimed he had yet to see a replay of either of his side's controversial goals but paid tribute to Wigan, saying: I think that Wigan were really unlucky not to get something out of this game. Because they played very well, they put a great performance in."