He was substituted after coming on against Southampton and started on the bench against Aston Villa on Saturday – days after being sent off for Serbia.
Mourinho, whose team face Dynamo Kiev in the Champions League on Tuesday, said Matic was lacking belief but hopes to rebuild it by returning to basics.
“Confidence is fundamental. Matic lost an easy pass when he came on [against Villa] and the first reaction he had was to show disappointment,” he said.
“When you are full of confidence you don’t show disappointment. You make a mistake, you know that you’re not going to make another mistake after.
“So [be] calm, let’s go step by step.”
Matic came on for Ruben Loftus-Cheek in Chelsea’s much-needed 2-0 victory against Villa.
Mourinho warned the 19-year-old there were still areas of his game that needed work.
“I think he [Loftus-Cheek] has to look to the intensity and the defensive tactical work that people like Ramires, [Cesc] Fabregas did and he didn’t,” he said.
“And try just to compare himself with others and understand why I didn’t keep him in the game for 90 minutes.”