De Jong has just had a bit-part role this term.
Mancini said: "Nigel had an injury for the first two months and couldn't play. After that we played very well with Gareth Barry and Yaya in the middle.
"I don't think he is happy but the player who is happy if he is not playing should not exist. Nigel is a serious professional and when he plays I am sure he will play very well."