The 23-year-old avoided serious ligament damage but suffered bruising and swelling to his ankle which is likely to rule him out of the last-16 clash at Old Trafford on Thursday night.
Ole Gunnar Solskjaer hopes that Rashford will be in contention to play this weekend but will have to manage without him against the Serie A title contenders.
“I don’t think he’ll be available for tomorrow,” Solskjaer said when asked about Rashford’s availability.
“He’s still getting treatment but he didn’t join in the training today. Even though normally I’m a very optimistic guy, I still think he’s going to be out tomorrow night and a doubt for the weekend.”
“That 60-70 yard recovery run [against City, which led to the injury] just showed the hunger and spirit and the humility in our work that we’ve got at the moment.
“It was probably worth it there and then, but you’re never happy when you lose players for games.”
United are also likely to be without Edinson Cavani, who missed Sunday’s derby through injury, while Donny van de Beek, Juan Mata and Paul Pogba also remain sidelined.
David de Gea is not available after returning to his native Spain to attend the birth of his daughter Yanay, leaving Solskjaer with no returning options.
“I’m not sure if anyone will be back,” he said Solskjaer. “David is not available. Donny, no. Paul, no. Edinson, maybe, but probably not.
“Juan, no and Marcus, maybe, but probably not. So it’s similar to Sunday, but maybe we’ve lost Marcus.”