Ronaldo was missing for Thursday's game with Aston Villa in the EFL Cup due to illness, while a health issue has also ruled Sancho out of the team's last three games in all competitions.
Antony has been out since the end of October with a hip problem, but Red Devils boss Erik ten Hag said on Thursday that he was hopeful of having the Brazil international available for this game.
According to the Manchester Evening News, Ronaldo did not board the train alongside his teammates on Saturday, while Sancho and Antony were also absent from the travelling party.
As a result, the trio could all again be unavailable for selection against the Cottagers.
Diogo Dalot will definitely be absent against Marco Silva's side due to suspension, which could see either Tyrell Malacia or Victor Lindelof play at right-back on Sunday afternoon.
Meanwhile, Raphael Varane, Brandon Williams and Axel Tuanzebe will be missing for the 20-time English champions due to injury.
Zidane Iqbal, Shola Shoretire and Alejandro Garnacho are all reportedly in the squad for the clash at Craven Cottage, where Man United will be bidding to return to winning ways in the Premier League.
Fulham have impressed in England's top flight this season, picking up 19 points from their 14 matches to sit in ninth position in the table.
However, Silva's side will be without the services of Aleksandar Mitrovic through injury on Sunday, while key duo Kenny Tete and Harrison Reed are suspended.
Speaking during his pre-match press conference, Man United boss Ten Hag said that he is expecting "a really tough game" against the capital side.
"That is going to be a really tough game. I think in the Premier League everything is very close, and what I have seen from Fulham so far is really energetic," the Dutchman told reporters.
"They also play really decent football out from the back - I remember the game against Liverpool at the start of the season. So you have to be really good there to get a result. We have to do everything in our power to get the right result - that is what we're preparing [for] now."
Man United are currently fifth in the table, four points behind fourth-placed Newcastle United, boasting a record of seven wins, two draws and four losses from their 13 league games this term.