The Blues stopper saw his side whitewashed in Rome and now faces another tough task against the Red Devils who line up with a former team-mate.
Thibaut Courtois and Nemanja Matic won two Premier League titles together but they will line up against each other on Sunday and the Chelsea goalkeeper understands why his club sold their midfielder.
Matic joined Manchester United for £40 million this summer and he has been in good form ever since his move to Old Trafford, while Chelsea's midfield has struggled in his absence.
Chelsea's sale of a regular starter from last season to a rival has seen a negative response, particularly as their form has dipped this month, with their most recent defeat coming in a 3-0 away loss to Roma.
Courtois thinks that Chelsea have sold a quality player in Matic but he understands why they chose to let him go.
"I don't know. I think we know he has qualities, he is someone that last year, give us the balance as well," Courtois said at the Stadio Olympico. "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.
"I don't know if we miss him but obviously, 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. That's not my decision to make [about selling him].
"Those are questions that you need to ask to the people who made that happen. I think that if a player nowadays wants to go to a team, the player has a strong will to join another team, it is hard and you cannot just ignore him and say you can't leave.
"Then you have a disappointed player who maybe won't play at his best. For us, you have given another player to another big teams. These are the choices made in football. He is a quality midfielder but hopefully we can show that we are better on Sunday."
Chelsea have struggled to show the same balance in their side that they showed last season when they won the Premier League and racked up a 13-game winning run in the winter period.
N'Golo Kante's injury has rocked Chelsea and Danny Drinkwater is still being eased back into the squad after a prolonged period of injury. Courtois acknowledges that Chelsea's midfield is part of the problem at the moment and he thinks that Cesc Fabregas and Tiemoue Bakayoko are growing tired.
"This season we are conceding more chances than we did last year," Courtois added. "We have to try to get that extra step in, that extra per cent to stop actions, not be 90 per cent. Maybe with a lot of games, sometimes you maybe go a bit less.
"Obviously it is different to last season. Injuries have been coming and not at a good time. We know how important N'Golo is for us, he is the guy that brings a lot of balance into the team. But we can't push him to play if he doesn't feel ready.
"Danny (Drinkwater) was also out for a long time as well. It means Tiemoue Bakayoko and Cesc Fabregas have played a lot of games in midfield and obviously that goes into their legs and it is harder for them to keep up the pressure, especially against the midfield three like Roma had last night".
