Pedro's quick fire opener for Chelsea was followed by further goals from Gary Cahill, Eden Hazard and N'Golo Kante as Chelsea thrashed Manchester United 4-0 Sunday in the English Premier League.
Mourinho, who enjoyed such success at Chelsea in his two spells at the west London club, could only sit gloomily on the touchline as his new side stumbled to a dismal defeat.
United had come into the heavyweight clash off the back of a fighting goalless draw at Liverpool Monday
and a Europa League thrashing of Fenerbahce Thursday, but always looked off the pace.
The tone was set from the moment Chris Smalling made a hash of clearing a hopeful long ball forward and Pedro nipped in to round David de Gea and put Chelsea ahead before fans had barely settled in their seats.
Zlatan Ibrahimovic headed over for the visitor before Mourinho's men found themselves further behind on 22 minutes as Gary Cahill volleyed home from close range after a corner.
Good fortune appeared with Chelsea as David Luiz crashed into Marouane Fellaini but avoided a red card which might have changed the course of the match.
Mourinho tried to as he reshuffled his team and bought on former Chelsea man Juan Mata after the break, but it was to no avail.
More slack defending allowed the influential Eden Hazard to evade Smalling and strike home the third just after the hour mark.
Summer signing from Leicester Kante completed the rout with another fine goal with 20 minutes remaining, his first for the Blues, who are on a run of three straight EPL wins under Antonio Conte and moved to fourth in the standings.
It was the heaviest defeat suffered by a Mourinho-managed team in the EPL and Chelsea's biggest win over the Red Devils since 1999.
"We made incredible defensive mistakes, individual defensive mistakes and you pay for that," Mourinho told Sky Sports.
"It started badly after a few seconds.
"You come with a strategy, you can not concede a goal in the way we did."
Man City winless run continues
Earlier Sunday, Manchester City went to the top of the EPL on goal difference, despite being held 1-1 at home by Southampton.
It was the fifth game without a win for the early runaway leader in the standings, equaling the worst run of results for its manager Pep Guardiola.
A dreadful mistake by England international John Stones handed Saints the opener on 27 minutes as Nathan Redmond converted.
Stones failed to make the clearance and his pass found Redmond to round Claudio Bravo to score.
City did not have a single shot on target in the first half, but improved after the interval as substitute Kelechi Iheanacho equalized from Leroy Sane's low cross.
Saints goalkeeper Fraser Forster made some key saves to leave Guardiola to reflect on a matching an unwanted record, stretching back to seven years ago with Barcelona.
"You can win 10 times and then you are not able to win five times. I have to discover the reason why and I am going to find that," he told the official Manchester City website.
But the point did take his team above Arsenal and Liverpool in the standings, with just one point separating the top five teams. Improving Southampton, unbeaten in five, stayed eighth.
Real goes top in Spain
In Spain's La Liga, Real Madrid salvaged a last-gasp 2-1 victory over Athletic Bilbao in the Bernabeu to move to the top of the standings.
Real took advantage of an earlier 1-0 defeat to challenging Sevilla with Steven N'Zonzi hitting the winner 17 minutes from time.
Real leads Sevilla by a point and Barcelona and Villarreal by two at the top of the table.
Madrid made the perfect start as Enric Saborit's slip allowed Isco to set up Karim Benzema for a seventh minute opener.
But Real's defensive frailty was exposed when Merino leveled for the visitor midway through the half.
With Cristiano Ronaldo misfiring at the other end, Athletic had its chances and Inaki Williams blazed over with the goal at his mercy in the second half.
Substitute Alvaro Morata rescued the home side, connecting with a Gareth Bale cross to score the winner and take his side to the summit.