The England U19 midfielder started in the Blues' Premier League game with Liverpool, almost 17 years since John Terry became the last homegrown Chelsea player to secure a regular first-team spot.
And in a campaign when Chelsea’s youngsters have won both The FA Youth Cup and UEFA Youth League, Mourinho believes Loftus-Cheek’s elevation is something to be proud of.
Loftus-Cheek in action against Liverpool on his Premier League debut
"This year, the most important title for the academy was Ruben, not the European or FA Youth Cup. It was Ruben," said Mourinho, who claims Loftus-Cheek will be a full-time member of his squad next season, regardless of any other incoming transfers.
"Maybe he is better than the player I buy.
"And if he is, he plays and the player I buy doesn't. But at the moment he's not better."
The Blues travel to face West Bromwich Albion on Monday evening and Mourinho has also revealed that there will another debutant for the champions at the Hawthorns.
England U19 forward Izzy Brown, who Chelsea bought from West Brom two years ago, will line-up against his old club.
And the Portuguese is looking forward to seeing the 18-year-old in action.
Izzy Brown celebrates his goal against Shakhtar Donetsk in the UEFA Youth League Final in March
"Izzy Brown – he plays," revealed Mourinho on Sunday.
"I want him to be champion and I want him to have his debut for Chelsea in the Premier League at West Brom.
"It means double for him to play for Chelsea, to be champions and to play against his old club."
Brown and Loftus-Cheek are just two of the young players now knocking on the first-team door at Stamford Bridge.
England U18s striker Dom Solanke played in the Champions League earlier this season and Dutch defender Nathan Ake has been in and around the squad this season.
Watch highlights of Chelsea's FA Youth Cup win this season
With the Blues academy winning the last two Youth Cups, a record of four wins from the last six years, there is no shortage of promise in reserve.
But Mourinho is keen for the club’s youngsters to remain grounded and is not entirely comfortable with the trappings often lavished upon talented young players.
"In modern football, because of the agents and the parents, when the players are in the process of being almost there, they think they are already there," admitted the former Inter Milan and Real Madrid chief.
"They make the players think they have arrived when they haven't. They think about money before the career starts and everything gets very, very confused.
"That doesn't help the players. They need stability.
"To be in a big team and to reach the level to be playing regularly for the first-team, you need time and stability. We try to give that to our boys."
