Muntari was jeered by home fans during last night's 2-1 victory over Udinese, but was embraced by teammates when he was substituted.
Mourinho said: "His teammates were extraordinary and it's easy to see how united this squad really is. He is not 100 per cent fit, he made a few mistakes, but that's natural.
"I think a player needs to be strong when it comes to the fans. I prefer the stadium to support their players no matter what, but this is the Italian culture and he needs to play without feeling the pressure."