Their early season performances suggest Manchester United pulled off a masterstroke in signing Nemanja Matic from their rivals Chelsea.
Jose Mourinho had previously signed him for Chelsea from Benfica and splashed out £35m on bringing the Serbian midfielder to Old Trafford.
Winning two Premier League titles, Matic was a first team regular during his three-and-a-half year spell at Stamford Bridge and has played every minute in the Premier League since moving up north.
The 29-year-old Manchester United star will be returning to his old stomping ground when Chelsea host the Red Devils on Sunday afternoon.
Trailing Manchester City by nine point already, the Blues' Belgian goalkeeper Thibaut Courtois has recognised the importance of Sunday's game.
"Manchester City are already nine points ahead of us, Manchester United four and Tottenham one," Courtois said.
"We must win games like these to remain at the top and close to the other ones. We will try to get the three points and will give our lives for it on Sunday."
Mourinho "surprised" to be offered Matic
Chelsea fans - and quite possible Antonio Conte - were completely bemused by the club's decision to sell one of the first names on the teamsheet to a direct rival.
Mourinho, speaking ahead of the big game, suggested Manchester United had not even considered moving for Matic, admitting that he was "surprised" to receive a phone call from the player's agent.
"I was surprised when [Matic's] agent called me to say 'do you want him?' " Mourinho told Sky Sports News.
"I was surprised, but he's an agent I know well. Normally he's very direct and objective, he doesn't waste time, he doesn't try to get people to be interested in his players to improve their situation at their club.
"Normally he's very straightforward, so when he told me 'you can have him if you want', of course I want."
Chelsea had seemingly authorised Matic's sale and gave the £35m transfer the green light once the Serbian's agent found him a suitor.
Phil Neville believes Conte's title winners are struggling because of Matic's departure and described his sale as "one of the poorest decisions I have ever seen in the Premier League."
"What's gone wrong for Chelsea? One word. Matic," Neville said. "It's as simple as that. The minute they sold Nemanja Matic was a mistake.
"When you had Matic sat in front of you, alongside [N'Golo] Kante, there was protection. Whoever made that decision needs sacking. That is one of the poorest decisions I have ever seen in the Premier League."
"If you ask any of those Chelsea players who they would want back, they would say Matic. Chelsea replaced him with someone who is going to do the same job – [Tiemoue] Bakayoko is not that player, he is not a holding midfield player.
"They didn't replace Matic. I wouldn't have let him out of the building, I would have chained him to the training ground walls."