Russian billionaire Abramovich twice hired Mourinho, in 2004 and 2013, and watched him become the club's greatest manager over his two spells at the helm, notably winning three Premier League titles.
But he has also sacked him twice, most recently last December, and ahead of the Portuguese's return to Stamford Bridge with United, Mourinho revealed his relationship with his former boss was only ever professional.
"He was never my friend. We had always the relation of owner-manager, a very respectful relation," Mourinho said.
Despite Chelsea's dismal title defence last season, which saw them slump to 16th place in the table, Mourinho is likely to receive a warm reception from the Stamford Bridge faithful.
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But with United two points below Chelsea and five points off leaders Manchester City in seventh place, Mourinho says he will have more important matters on his mind.
"I'm more focused on the game. I try to prepare myself for the matches, so I cannot say that I care. What can I expect? I don't know," he told reporters after United's 4-1 Europa League win over Fenerbahce on Thursday.
Mourinho has admitted he will no longer feel a sense of
invincibility when he returns to the Bridge.
He avoided a home league defeat in his first three-year stint at Chelsea and prior to a 1-0 Champions League win there with Inter Milan in March 2010 he said the stadium was "lucky" for him.
"When I played there with Inter, before that I never lost a match at Stamford Bridge because I had that home record that is still in the Guinness (World Records)," Mourinho said.
"Now I lost already a few matches at the Bridge. Last season I lost two or three matches, so I cannot use the same words because now I lost there."
United have already lost to Manchester City and Watford this season and dropped points in draws against Stoke City and Liverpool in their two most recent outings.
"We are in a period where some teams are having easier fixtures," he said.
"Maybe apparently some teams are going (well), but in a couple of weeks they play between them. So my feeling is that it will be really tight. One team that you think is playing for the title in March, probably in May doesn't finish in the top four.
"I'm not a gambler, but if I was I wouldn't bet a lot on the title or the top four because it's a big risk to lose money.