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.
Chelsea host Manchester United on Sunday and even though David Moyes' side trail leaders Arsenal by 11 points, Mourinho refuses to rule them out of the title race.
"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."
Mourinho insists United remain dangerous opponents even though they sit seventh in the table, five points adrift of the Champions League places.
"We play against the champions. We play Manchester United, not a team 11 points behind the leaders," he said.
"I've played many big matches and this is what my experience tells me.
"A team that appears to be in the most difficulty can be the most dangerous."
Source : PA