Only a month ago, Terry feared the worst as he was increasingly marginalised by interim manager Rafa Benitez, who often left him out of the biggest games.
Mourinho said this week: "I like the fact that we still have four or five players from the beginning of the winning Chelsea.
"It's very important for this football club to always have them as a part of the Chelsea soul."
A club source told The Sun: "No-one is happier about Jose's return than John.
"He didn't get on with Benitez and believes the change of manager will see him back to his best next season."