The 22-year-old's influence was so great early in the season that he was chosen as Mourinho's playmaker and Juan Mata - Chelsea's player of the year in the past two seasons - was marginalised and eventually sold to Manchester United.
The Brazil forward has struggled to continue making the same telling contributions in recent weeks, with Eden Hazard Chelsea's standout performer.
Mourinho, speaking ahead of Saturday's Premier League clash with Everton, said: "Maybe in the last couple of matches he was not the same, because in the beginning of the season he was the best player in the team, he was the player that was really making the difference in the first three, four months of competition.
"In this moment he's not being a match-winner, he's not scoring goals, but he's not playing bad. Not real problems."
Oscar is a "fantastic player" with "all our trust", added Mourinho, who offered an explanation for the dip in performance.
"The talent is there all the time, but the physical top condition to express the quality he has is not always there," Mourinho added.
"We have to take care of him. He's a young boy. He's not a natural physically strong man, suffers with the accumulation of matches, suffers with the difficult weather we had."
Perhaps Oscar's issues related to last season, when his campaign began at the London 2012 Olympics, he played in nearly all Chelsea's 69 competitive games and then he continued on to the 2013 Confederations Cup.
Mourinho, though, believes it is all part of the maturing process.
"I can imagine Oscar in three, four, five years' time, at 25, 26 years old," Mourinho said.
"We all can imagine. In this moment it's different. He's a young guy."