Pep Guardiola may have thought it a bad day when he suffered a fifth match without a win on Sunday but it was nothing compared to the nightmare that befell his old managerial rival Jose Mourinho on an extraordinary afternoon of Premier League action.
Guardiola's Manchester City went back to the top of the table on goal difference after a 1-1 draw at home to Southampton, leaving him to reflect glumly on a five-game winless streak for only the second time in his garlanded career.
A couple of hours later, though, his Manchester United rival Mourinho, a general not used to humiliation, was forced to sit at a ground he once commanded and see his side ripped apart 4-0 as Chelsea produced one of the standout displays of the season at Stamford Bridge.
The results saw the battle at the summit hot up with only one point now separating the top five.
City lead Arsenal and Liverpool, with all three teams on 20 points, while Chelsea's triumph shoots them up to fifth, level on 19 with fourth-placed Tottenham Hotspur.
United, though, are now seventh on 14 points, one adrift of Everton, and with the pressure mounting on Mourinho, Wednesday's League Cup clash with Guardiola's men at Old Trafford takes on huge significance.
This was only the second time that a Mourinho team have conceded four goals in the Premier League.
For that to happen at Stamford Bridge was unthinkable for the Portuguese, who began the day with a hug from John Terry and ended it with what appeared some bitter words in the ear of his opposite number Antonio Conte.
United had just been ripped apart with goals from Pedro - the quickest in the Premier League this season - Gary Cahill, Eden Hazard and N'Golo Kante, and, as the clock ticked down, the Italian gesticulated to the crowd for even more noise.
Chelsea's N'Golo Kante celebrates scoring against Manchester United with team mates
ReutersNeither manager would reveal what was said in the post-match exchange but Conte said he was entitled to act as he did. "If you want to cut my emotion, then I go home and change my job," he said.
Having held out for 90 minutes in the goalless affair at Liverpool on Monday, United fell behind after just 30 seconds when a range-finder from Marcos Alonso found Pedro in space behind United's defence.
It set the scene for a woeful United defensive performance that angered Mourinho, who was forced to watch real excellence from the same Chelsea players who had failed to perform for him in the build up to his sacking last December.
"We made incredible defensive mistakes and you pay for that," said Mourinho, whose only previous Premier League experience of seeing his side leak in so many goals was when Chelsea were beaten 5-3 by Tottenham Hotspur in 2015.
Chelsea manager Conte and Manchester United manager Mourinho at the end of the match
Reuters"If we score a goal like we almost did for 2-1 the game would be different," added Mourinho, whose day was made worse by a serious knee ligament injury to defender Eric Bailly.
Wednesday's match should prove some managerial joust with Guardiola, who won their first Manchester derby, also sounding a mite desperate to get back on the winning trail again.
He knows how Mourinho feels, having gone back to Barcelona in midweek and suffered a similar 4-0 trouncing.
City had to come from behind after John Stones gifted Southampton the lead with a misplaced pass to Nathan Redmond.
Manchester City's David Silva in action with Southampton's James Ward-Prowse
ReutersThe draw, secured by Kelechi Iheanacho's second-half equaliser, means City, who started life under Guardiola with 10 straight wins, have now not won for five games. It equals the Spaniard's worst run as a manager when he was at 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," Guardiola vowed.
RESULTSChelsea 4 Pedro 1, Cahill 21, Hazard 62, Kante 70
Manchester United 0Manchester City 1 Iheanacho 55
Southampton 1 Redmond 27
Standings P W D L F A
Pts1 Manchester City 9 6 2 1 20 9
202 Arsenal 9 6 2 1 19 9
203 Liverpool 9 6 2 1 20 11
204 Chelsea 9 6 1 2 19 9 19
5 Tottenham Hotspur 9 5 4 0 13 4
196 Everton 9 4 3 2 13 8
157 Manchester United 9 4 2 3 13 12
148 Southampton 9 3 4 2 11 8
139 Watford 9 3 3 3 13 13
1210 Bournemouth 9 3 3 3 12 12
1211 Crystal Palace 9 3 2 4 12 12
1112 Leicester City 9 3 2 4 11 15 11
13 West Bromwich Albion 9 2 4 3 10 10
1014 Burnley 9 3 1 5 8 13
1015 West Ham United 9 3 1 5 10 17
1016 Stoke City 9 2 3 4 9 16
917 Middlesbrough 9 1 4 4 7 11
718 Hull City 9 2 1 6 8 22
719 Swansea City 9 1 2 6 8 15
520 Sunderland 9 0 2 7 6 16
2 
Chelsea's N'Golo Kante celebrates scoring against Manchester United with team mates ReutersNeither manager would reveal what was said in the post-match exchange but Conte said he was entitled to act as he did. "If you want to cut my emotion, then I go home and change my job," he said.
Having held out for 90 minutes in the goalless affair at Liverpool on Monday, United fell behind after just 30 seconds when a range-finder from Marcos Alonso found Pedro in space behind United's defence.
It set the scene for a woeful United defensive performance that angered Mourinho, who was forced to watch real excellence from the same Chelsea players who had failed to perform for him in the build up to his sacking last December.
"We made incredible defensive mistakes and you pay for that," said Mourinho, whose only previous Premier League experience of seeing his side leak in so many goals was when Chelsea were beaten 5-3 by Tottenham Hotspur in 2015.
Chelsea manager Conte and Manchester United manager Mourinho at the end of the match Reuters"If we score a goal like we almost did for 2-1 the game would be different," added Mourinho, whose day was made worse by a serious knee ligament injury to defender Eric Bailly.
Wednesday's match should prove some managerial joust with Guardiola, who won their first Manchester derby, also sounding a mite desperate to get back on the winning trail again.
He knows how Mourinho feels, having gone back to Barcelona in midweek and suffered a similar 4-0 trouncing.
City had to come from behind after John Stones gifted Southampton the lead with a misplaced pass to Nathan Redmond.
Manchester City's David Silva in action with Southampton's James Ward-Prowse ReutersThe draw, secured by Kelechi Iheanacho's second-half equaliser, means City, who started life under Guardiola with 10 straight wins, have now not won for five games. It equals the Spaniard's worst run as a manager when he was at 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," Guardiola vowed.RESULTSChelsea 4 Pedro 1, Cahill 21, Hazard 62, Kante 70 Manchester United 0Manchester City 1 Iheanacho 55 Southampton 1 Redmond 27Standings P W D L F A Pts1 Manchester City 9 6 2 1 20 9 202 Arsenal 9 6 2 1 19 9 203 Liverpool 9 6 2 1 20 11 204 Chelsea 9 6 1 2 19 9 195 Tottenham Hotspur 9 5 4 0 13 4 196 Everton 9 4 3 2 13 8 157 Manchester United 9 4 2 3 13 12 148 Southampton 9 3 4 2 11 8 139 Watford 9 3 3 3 13 13 1210 Bournemouth 9 3 3 3 12 12 1211 Crystal Palace 9 3 2 4 12 12 1112 Leicester City 9 3 2 4 11 15 1113 West Bromwich Albion 9 2 4 3 10 10 1014 Burnley 9 3 1 5 8 13 1015 West Ham United 9 3 1 5 10 17 1016 Stoke City 9 2 3 4 9 16 917 Middlesbrough 9 1 4 4 7 11 718 Hull City 9 2 1 6 8 22 719 Swansea City 9 1 2 6 8 15 520 Sunderland 9 0 2 7 6 16 2 