The Red Devils are without the services of star midfielder Paul Pogba after the France international limped off in their 3-0 Champions League win over Basel. The former Juventus man has been partnering United's summer signing in the heart of the midfield for league fixtures.
Jose Mourinho fielded Fellaini alongside Matic due to Pogba's absence. The Portuguese tactician has "no idea" when the midfielder would return and the decision is yet to be made whether to let the France international have a surgery on his hamstring injury.
United have Ander Herrera, Michael Carrick and 20-year-old Scott McTominay as the other midfielders in the squad. Robson believes his former club can cope with injuries, but remains hopeful that Matic and Fellaini are not long term absentees.
"Injuries will be key. The squad is strong but Pogba has his hamstring problem while Matic and Marouane Fellaini picked up knocks against Southampton. United can cope but won't want too many injuries in the same area of the pitch, especially one as crucial as central midfield," Robson told the Daily Mail.
"You would have concerns if Lukaku picked up an injury, too, but Rashford and Martial are sharing the scoring responsibility and, fingers crossed, Zlatan Ibrahimovic will be back in December."
United have enjoyed a good start to the 2017/18 campaign and are level on points with leaders Manchester City. Robson believes Mourinho's men are capable of challenging Pep Guardiola's and Chelsea for the title this season.
"It's been four years since United won the league. Chelsea look strong and City are a great attacking threat but United certainly have nothing to fear," the former United captain explained.