The 20-year-old did not train with the Red Devils last week and did not make the trip to Vicarage Road for the fixture with the Hornets on Saturday afternoon.
Speaking ahead of the contest, Man United boss Ole Gunnar Solskjaer said that the attacker is now facing a period of self-isolation after testing positive for the illness.
"Mason has got COVID, unfortunately," Solskjaer told MUTV. "He's tested positive, so we'll wait for him to get back."
Greenwood has been a regular for the 20-time English champions during the 2021-22 campaign, making 14 appearances in all competitions, contributing four goals and one assist.
In his absence, there is a spot in the first XI for Jadon Sancho, while Marcus Rashford, Bruno Fernandes and Cristiano Ronaldo will also feature in the forward areas for the visitors. body check tags ::