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."
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."
For Bertrand, joining Villa represents his eighth loan spell away from the club since signing for Chelsea as a 16-year-old in the summer of 2006.
In his seven-and-a-half years at Stamford Bridge, Bertrand has only made 57 appearances, although one of those was as a starter in the 2012 Champions League final against Bayern Munich.
Bertrand has also represented England at every level from the under-17s, with two senior caps to his name at the start of last season, as well as playing for Great Britain in the 2012 Olympics.
But with just three appearances for Chelsea this season, Mourinho has decided Bertrand's short-term future lies elsewhere, with Villa only too happy to acquire his services to strengthen their defence.
Speaking to Villa's website, manager Paul Lambert said: "Ryan brings a lot of big-game experience, not only in the Barclays Premier League but even a Champions League final which Chelsea won, so he'll be really good for us.
"The way he plays the game and as a person too, he'll fit in here great and it will be good for him also to be playing games.
"He has real enthusiasm for the game and he's looking forward to playing here.
"We're grateful to Chelsea for allowing him to come here on loan and we know he'll be really good to work with."
Bertrand knows it is more upheaval to his career, but he said: "I've had to adjust many times in my career having played under several managers, so I know I can settle in here quickly and I'm looking forward to being able to contribute to the team.
"I'm focused on continuing to progress and helping the team here to do that too.
"We have a lot of young, talented players here with some experienced heads as well, they want to express themselves and I hope I can add to that and add my experience in a productive way.
"It's a fantastic opportunity for me to come here. Aston Villa is a fantastic club with a massive fanbase and a great history.
"To get this chance to come here and play some games will be a really good step for me."
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