The Red Devils were crushed at Stamford Bridge as Mourinho returned to his former club for the first time as United manager. Pedro's goal after just 30 seconds began a rout hallmarked by defensive errors from the away side.
Mourinho told MUTV there was only one way to resolve the issues that were so apparent in west London (via the Sun's Claire Bloomfield):
They kept going for 90 minutes in an unbelievable way, the fans that we have all around the world they are obviously with a very bad feeling and I feel so sorry for them.
I have to apologise for that and the only thing I can say is that I am Man United 100 per cent, not 99 per cent and 1 per cent for Chelsea or any other club and because of that I feel deeply the situation.
But there is only one answer, like I was telling the players, there is only one answer—training tomorrow and we fight on.
United are now six points behind Premier League leaders Manchester City, having failed to win any of their last three league matches.
However, Pedro's early goal put paid to the Portuguese manager's plans, as the Spain international took advantage of Chris Smalling's hesitation and mistakes from Daley Blind and David De Gea.
Failure to clear from a corner saw Gary Cahill put Chelsea 2-0 up after 21 minutes, and the Blues' second-half goals from Eden Hazard and N'Golo Kante could also have been prevented had United's defenders been more mobile.
Mourinho was all too aware of his side's issues after the match.
"It is one of those days when you give the advantage to opponents by doing nothing," he told Sky Sports (via Bloomfield). "We made incredible defensive mistakes, individual mistakes. And you pay for that."
Former United player Phil Neville insisted Mourinho should be absolved of some of the blame for Sunday's defeat as he could not legislate for his players' errors, per BBC Sport:
Because of City's recent poor form, United could re-establish themselves as title contenders if they can string a few wins together.
A victory against the Clarets on Saturday is an absolute must if United are to regain their confidence following Sunday's debacle in the capital.