Luke Shaw looks unlikely to play again this season after he broke his leg in two places during Manchester United's 2-1 defeat to PSV Eindhoven.
Shaw was carried off on a stretcher while receiving oxygen after a tackle from behind by Hector Moreno in the 15th minute.
United confirmed the left-back's injury after the match.
A United spokesperson said: "Luke Shaw has suffered a double fracture of the leg."
Shaw will now be transferred to Manchester to be operated on.
With the score at 0-0, the United left-back was taken out with a sliding tackle by Moreno in the 15th minute of the group stage match in Holland.
Replays showed Moreno had swept both of Shaw's feet from underneath him and his right foot lay at an angle, which indicated a break.
He stayed down for eight minutes of treatment before being carried off on a stretcher while receiving oxygen. Louis van Gaal looked upset at seeing the 20-year-old being carried off on a stretcher.
The United manager went over to the defender and gave him a pat on the shoulder as he was carried down the tunnel. He was then taken to hospital. Both sets of fans applauded as Shaw left the field.
Thank you everyone for your messages , words can't describe how gutted I am , my road to recovery starts now, I will come back stronger.
— Luke Shaw (@LukeShaw23) September 15, 2015
Moreno was not penalised for the tackle, which happened in the PSV penalty area.
Marcos Rojo, who replaced Shaw at the Philips Stadion, is likely to be a long-term replacement for Shaw at United this season.
Shaw's team-mate Adnan Januzaj, currently on loan at Borussia Dortmund, wrote on Twitter: "Horrible to see what happened ... I wish u all the best my brother and stay strong you will comeback even stronger ... @LukeShaw23"
Shaw himself took to Twitter, as many well-wishers sent him notes of support.
He wrote: "Thank you everyone for your messages , words can't describe how gutted I am , my road to recovery starts now, I will come back stronger."
His former club Southampton told Shaw to "come back stronger", to which he replied: "thank you , you don't know how much you as a club mean to me , thank you so much I really appreciate it"
