A knee injury ruled him out for five weeks. But after recovering, Ancelotti failed to consider him for crucial matches.
Mikel, 24, said: "I had a good start to the season until about January. Then when the club had a bad run, it affected everybody. Everything went downhill.
"I was playing in every game, Champions League and Premier League, but suddenly we had this bad run which went on for about a month or two.
"I had a knee injury in the middle of it as well and was out for four or five weeks.
"When I came back, I wasn't playing regularly - just a few games. It was frustrating for me.
"It's very difficult as a young player but I've learned to deal with it.
"Every season you are going to have a period when things are not going well for you and other times when things are going well. You have to try and manage both.
"I just try to stay professional, train and forget about it. I play when I get the opportunity."