The Belgium international used that same word to describe his role under Mourinho, stating that his ability to help out in a number of positions makes him indispensable to his manager.
Asked what qualities of Lukaku's were so vital to him, the former Real Madrid and Chelsea chief cited an example from Monday night's comeback win at Crystal Palace.
"He's an important guy for me, one of the guys that I trust," he told reporters at his pre-match press conference ahead of Saturday's meeting with Liverpool. "I love the attitude, I love the character.
"Against Palace, there was nothing that happened after our third goal but I told him 'central defender!', so after the third goal he was playing central defender in a back five with [Chris] Smalling and [Victor] Lindelof because I couldn't make any more changes.
"So when I say to the striker go there and do a job for me he is a sergeant. He is a guy you can trust, his attitude is phenomenal. That is why I always thought and told you many times for me it's not just about the goals he scored but what he brings to the team and he brings to the team that exact spirit."
Lukaku ended a goalless run spanning 10 games in meetings with fellow top-six opposition by finding the net against former club Chelsea last month.