The Red Devils host Watford in the third round of the FA Cup on Saturday and the 19-year-old was hoping to make his debut after his summer move from Uruguayan club Penarol.
However, his talent agency has confirmed that he has tested positive for coronavirus and, although he is not suffering with any symptoms, he is isolating at home.
‘He is doing the corresponding quarantine at home, without symptoms and complying with the physical care indicated by his club.
‘We appreciate the messages received in the last hours. Soon he will be training again with his teammates.’
Pellistri has been training with the first team, but as yet there have been no further reports of positive tests emerging from Old Trafford.
The teenager, who arrived for a fee of around £10m, has been helped in his United career so far by compatriot Edinson Cavani, who also signed for the club over the summer.
‘It’s been amazing to train with guys like [Marcus] Rashford, [Paul] Pogba, [Juan] Mata and Edi [Cavani], who is very close friends with me. It’s amazing for me because I watched them on TV and now I am practising with them, he told MUTV.
‘The intensity, the high quality they have, it’s great for me to play with them and I improve a lot.
‘He is there to help me and that is very important for me. I am very happy with that.
‘I know he is one of the best players in the world, and one of the all-time best players from Uruguay, but he is also a very nice person.’