Cesar Azpilicueta and Ashley Cole are the Blues' preferred left-backs so Bertrand, who has won two England caps, has agreed to join Villa in search of first-team action.
De Bruyne could follow him out of west London with German club Wolfsburg reported to be closing in on a £16.7million move for the Belgian midfielder.
The return of Nemanja Matic has freed up space in Chelsea's squad and Mourinho has sanctioned De Bruyne's departure.
"We decided the best thing for Ryan was to go on loan until the end of the season because he's a good player, he's our player and he needs to play," Mourinho said.
"My options at left-back are Cole and Azpilicueta. So for Ryan we felt, and he agrees totally, that he needs to play.
"He's young and English and needs to play, so he's going to Aston Villa until the end of the season.
"Kevin is more than probably leaving. It's close."
Mourinho has ruled out making any further signings during the transfer window following the arrival of holding midfielder Matic.
"It's more about people leaving than coming in. I'm happy with the squad," Mourinho said.
"We are building a squad not just for now but also for the future.
"Matic was a player we didn't want to lose out on. If we didn't do it now, we would have lost out."