The Serbia international's transfer has been one of the most discussed of the summer, with the Blues facing criticism from both pundits and fans as they suffered from inconsistent form during the month of October.
Chelsea decided to sell Matic to United for £40 million after talks stalled on a new deal for the 29-year-old, who had less than two years to run on his contract.
He enjoyed life in London, where his family had settled well, and enjoyed his second spell at Chelsea. But he had his head turned by United boss Jose Mourinho and the Red Devils' willingness to give him a pay rise that Chelsea were reluctant to offer during contract talks.
Along with a meatier wage packet at Old Trafford, he was offered a greater role within the squad as he symbolically replaced an ageing Michael Carrick. He is now the central figure who provides his side with balance, a winning mentality and a strong work ethic.
Meanwhile, Chelsea had turned their attention to Monaco's Tiemoue Bakayoko. In fact, they had been chasing him long before the transfer window opened, impressed by the role he played as Leonardo Jardim's men stormed to the Ligue 1 title and the semi-finals of the Champions League.
The Blues ended up securing Bakayoko for nearly £40m as they ceded Matic, who might also have gone to Juventus, who had a long-term interest in him.
Indeed, Conte had been forced to keep the Serbia international midfielder the previous summer amid interest from Turin.
Juve wanted the 29-year-old again in 2017 but once Mourinho discovered the player was available, he did not hesistate to make his move.
Matic was training alone at Cobham Training Centre as his move neared and Bakayoko arrived at the club. The Frenchman had a knee injury upon signing, leading to the Blues starting the season with a depleted squad, which in turn led to criticism over their transfer activity. Diego Costa's status as an exile in Brazil heaped more negative attention on Chelsea ahead of his move to Atletico Madrid in September.
Ex-Manchester United academy player Danny Drinkwater came in on transfer deadline day for a fee of £35 million from Leicester City. However, he suffered a calf injury within a week of signing and his debut was delayed until nearly two months after he signed for the club. Although Bakayoko soon returned, N'Golo Kante then got a hamstring injury while away with France on international duty.
That has meant that Chelsea have gone into many games with only two fit senior central midfielders. Cesc Fabregas is beginning to look jaded having racked up an impressive 16 games already this season. Thibaut Courtois recently spoke at length about the sale of Matic and he believes that the injuries have led to Chelsea missing him more.
"He is someone who gave us balance last year," Courtois said in Rome . "He decided to go to another team, we won't live in the past. He decided to leave, he left. Our midfield is very good, we have strong players.
"If you see we have two injuries in the midfield, then obviously you could say we missed Nemanja. He is a quality player who is doing well at Manchester United."
Matic's transfer away was been heavily criticised by many pundits, especially after Chelsea's most recent 3-0 defeat to Roma in the Champions League.
"What's gone wrong for Chelsea? One word. Matic," Phil Neville told BBC 5Live . "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 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."
Matic's sale has undoubtedly boosted Man United and hurt Chelsea - but only in the short term. The Blues now have two younger midfielders in the first-team squad and believe that they have gone one step backwards to go two forwards as Bakayoko matures into his role. Drinkwater, meanwhile, could improve, too, as he returns to one of England's major clubs having been sold by Manchester United as a 21-year-old.
Chelsea won't be sacking anyone over the sale of Matic and they will be best judged at a later date over whether they made the right call in selling their experienced midfielder.