The Red Devils suffered a humbling 3-0 home defeat to Tottenham Hotspur in the Premier League on Sunday and salt was rubbed into their wounds when Kobbie Mainoo was forced off in the first half with a muscle injury, making him a doubt for the trip to Porto.
Injury-plagued Mason Mount was also taken off with a cut to his head, and while he has since posted on Instagram to clarify that he is "all good", the midfielder is set to be assessed ahead of kickoff along with Harry Maguire, who missed the defeat to Spurs with a knock.
Both Tyrell Malacia (knee) and Victor Lindelof (toe) have resumed first-team training after spells on the sidelines - the latter was an unused substitute on Sunday, while it remains to be seen whether the former will be ready to return to the matchday squad on Thursday. Leny Yoro (foot) and Luke Shaw (calf), meanwhile, remain in the treatment room.
Erik ten Hag, who once again finds himself under significant pressure, will consider freshening up his starting lineup on Thursday, although a back four of Noussair Mazraoui, Matthijs de Ligt, Lisandro Martinez and Diogo Dalot could remain intact.
In the potential absence of Mainoo, ex-Porto man Casemiro or Christian Eriksen could link arms with Manuel Ugarte in centre-midfield, while Bruno Fernandes - who was sent off in the defeat to Spurs - is available to continue in the number 10 role for this Europa League clash.
After wasting a few decent opportunities in the final third on Sunday, Joshua Zirkzee could be replaced up front by Rasmus Hojlund if Ten Hag feels that he is ready to start having only recently recovered from injury, while Amad Diallo and Antony will continue to battle with Alejandro Garnacho and Marcus Rashford for starts on the flanks.
Manchester United possible starting lineup: Onana; Mazraoui, De Ligt, Martinez, Dalot; Eriksen, Ugarte; Diallo, Fernandes, Rashford; Hojlund