The winger is on loan at Juventus until the end of the season, but the Italian club want to make the transfer permanent when the transfer window re-opens.
Chelsea had wanted around £18m for the former Fiorentina player, but could now be prepared to accept a lower offer of £16m to bring in some cash for transfers.
"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."
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