The 'Red Devils' were spotted at Manchester Airport on Monday morning as they started their 3,200 mile trip to Russia, though Fellaini was not among the travelling party.
Youngsters Scott McTominay, 20, and Axel Tuanzebe,19, were both present for the trip and could feature for Jose Mourinho's side in the top of the table Group A clash.
Nemanja Matic also picked up a knock in the win at St Mary's but has made the trip, whilst Eric Bailly could be in line for his Champions League debut for United.
With regards to Fellaini, United are though to be furious with the challenge that caused the Belgian's injury, believing that 'Saints' striker Shane Long should have been sent off for the tackle.
In the end, the Irishman was only booked, though according to reports in 'The Daily Mail' Mourinho's side believe that had Fellaini been the culprit, the outcome would probably have been more severe.
It is thought that the injury is not too serious, though United are not keen to put a timeframe on Fellaini's recovery for the time being.
We're off to Russia! #MUFC pic.twitter.com/3RIcXYkSSu
— Manchester United (@ManUtd) September 25, 2017
En route to Moscow with @Aeroflot. pic.twitter.com/VyYlFx9yXL
— Manchester United (@ManUtd) September 25, 2017