Mourinho revealed the 24-year-old left-back has agreed to the move due to his lack of first-team action at Stamford Bridge.
"Ryan is to go on loan for the rest of the season. We felt and he agrees totally that he needs to play," Mourinho said.
Mourinho also confirmed Belgian midfielder Kevin De Bruyne is close to leaving the Barclays Premier League title-chasers.
"This transfer window is more about people leaving than coming. We are happy with our squad," he said.
"Arsenal and Manchester City are the top two teams," he said.
"City have the ammunition and Liverpool have a fantastic chance because they are not in European competition.
"Man United are Man United. Eleven points is a big gap but I believe they'll push all the way."
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- Barclays Premier League
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- Manchester United