Terry was a key figure for Mourinho during a previous spell at Stamford Bridge, lifting five major trophies - including two Premier League titles.
The former England captain has, however, suffered a tough time in recent years - with interim coach Rafa Benitez dropping him down the pecking order in 2012/13.
Mourinho concedes that upon returning to the Blues last summer he needed to be convinced that Terry was still the talismanic figure he had placed so much trust in before.
He has been left in no doubt that that is still the case, with the 33-year-old becoming a key performer once more.
Terry could now be offered fresh terms at the end of the season, with the experienced centre-half having been a virtual ever-present throughout Chelsea's domestic and European campaigns.
Mourinho said after Saturday's 1-0 victory over Everton, in which Terry netted a stoppage-time winner: "I didn't know (what to expect). And I told him that.
"I told him I wanted to understand what had happened. Some players lose form, lose their performance level and lose condition.
"I wanted to feel if that was the case. I wanted to see if it (him not playing) was down to other managers' options. That is something you always have to respect. So I didn't know. I started pre-season with him, analysing the situation.
"With me he has always played well, except when he had back surgery and lost an important part of the season. But in my first two seasons here he had a fantastic level. Now? This season he is doing very well."