McLeish believes Blues' recent demanding schedule finally took its toll on his players in Tuesday's 2-0 home reversal against Newcastle in the Barclays Premier League.
The Blues manager will make several changes for this weekend's FA Cup tie with Sheffield Wednesday ahead of the Wembley date with Arsenal on February 27.
"It will be an opportunity for some players to be rested and recharge their batteries and try and get an edge back," said McLeish. "There was definitely a flatness about some performances the other night after a fantastic month.
"We will look at that closely and the FA Cup will allow us to utilise the squad. There are some fantastic players to come into the team.
"But I stress we still want to progress in the cup which is a fantastic competition.
"We made changes and played well in the last round against Coventry to come from two goals down to win."
Source: PA
Source: PA