The former midfielder believes the Blues have declined since the Italian left on New Year’s Day, despite the appointment of Liam Rosenior.
Although Rosenior enjoyed a positive start, recent poor results against Leeds and Burnley have raised fresh concerns.
Speaking on the Obi One podcast, the Nigerian said the club acted too quickly in removing a manager who had improved the team’s style of play.
"We had Enzo Maresca. I've said it, I think it was a massive mistake to sack him. I mean, it's ridiculous," Mikel said on the Obi One podcast.
"Yeah, a manager that won us the Club World Cup, won us the Conference League, we had some sort of identity the way we played. We took a step forward, and we've gone five steps backwards.
“Liam has come in, and he's won games, but there is absolutely something that is missing. Something is not quite right.”