Blues are only out of the Barclays Premier League bottom three on goal difference after losing 2-0 at home to Fulham, Brede Hangeland grabbing both goals.
They have won only two of 11 games since their Wembley win over Arsenal, while Lee Bowyer, Martin Jiranek, Stuart Parnaby and Alexander Hleb all limped off against the Cottagers.
McLeish said: "Maybe mentally since the Carling Cup final, it took its toll for a while. Then we picked ourselves up for a while with a couple of great victories against Sunderland and Bolton back-to-back.
"But we seemed short in terms of legs and presence as well. I think the suspensions and injuries has caught up with us a wee bit."
He added: "But once we lick our wounds over the next couple of days, we will be bang at it. We've got to believe and I've been in situations before where it's looked impossible and it's worked and happened for us."
Blues were again guilty of conceding sloppy goals but also looked devoid of inspiration.
"We were second best," McLeish admitted.
"Fulham were the better team and I'm disappointed we lost goals at set-pieces the way we did. They were two soft goals again really.
"You can say the first goal we conceded smacked of nerves but I didn't think they were nervous in the dressing room beforehand."
"It is a tough task for us to go to Spurs and win but we've done it before. The evidence is there it can be done and it is about picking ourselves up and getting off the floor because that dressing room is hurting in there. They know we now face a battle to retain Premier League status."