Pellistri made his first appearance in a United shirt on Friday, with the youngster offering glimpses of his talent in an Under-23s clash with Everton.
He was the subject of a number of tough challenges, with Beni Baningime and Seb Quirk both getting booked for tackles on Pellistri.
Asked about Pellistri's performance in his post-match interview, United's Under-23s boss Neil Wood said: "It was 'welcome to England!'
"But it was great for him to get 90 minutes on the pitch and a first taste of English football. I'm sure he can build from this experience."
Williams added: "Facundo's done very well since coming in. He doesn't speak that much English but he's learning. He'll kick on from this and I'll be here to help him if I can. I enjoyed playing with him."