Blues have slipped into the bottom three after a woeful performance in Saturday's 3-1 home reversal against fellow strugglers West Brom.
McLeish was deprived of several key players through injury, including midfield duo Barry Ferguson and Craig Gardner who will again be absent for tomorrow's meeting with Everton at Goodison Park.
McLeish, whose side entertain Bolton in the FA Cup quarter-finals on Saturday, said: "You definitely worry about burn-out because of the amount of games they've played.
"The concentration that was there for the Stoke game and then the Arsenal game at Wembley in the Carling Cup was missing at the weekend.
"Whether that was tiredness or burn-out, then that's the only excuse I would be prepared to make for them."
Source: PA
Source: PA