Di Matteo admitted the Blues suffered a hangover from Wednesday night's Champions League win over Benfica, despite making seven changes.
"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," he said.
"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."