Mourinho dropped Matic for their home clash with Southampton before the international break.
He brought him on in the second half only to sub him before full time.
He was on the bench again this week against Aston Villa before coming on at half time - with the Blues winning 2-0.
But Mourinho revealed how the 27-year-old has become fragile.
"Confidence is fundamental. Matic lost an easy pass when he came on, on Saturday, and the first reaction he had was to show a disappointment," Mourinho 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 (stay) calm, let's go step by step.
GETTY
Matic was subbed having been brought on against Southampton
GETTY
Matic takes his place on the bench
"When you are a little bit fragile, simple. Make it simple. Think quick, make the right decision. Take the ball out of the pressure zone. Don't be worried about making a beautiful pass.
"Make just sure that you don't lose the ball, because if you lose the ball, you lose confidence. So, you know, step by step, step by step.
Chelsea face Dynamo Kiev in a crucial Champions League clash tonight.
Invalid email
We use your sign-up to provide content in ways you've consented to and to improve our understanding of you. This may include adverts from us and 3rd parties based on our understanding. You can unsubscribe at any time. Read our Privacy Policy
Mourinho insists Chelsea 'must win games'
Mourinho's side won their opener but lost their second Group F game to Porto.
But the Portuguese is optimistic about his side's chances in all four cups still.
He added: "We can win all four, we can lose all four. I think it is (possible), but I think it's also possible to lose all four.
"We are in October. Everything is open. We can win all four, we can lose all four."
