Pellistri is currently on loan at Alaves in La Liga and is with the national team after gaining his first senior call-up for crucial World Cup qualifiers against Paraguay and Venezuela.
It will have provided a huge boost to the winger, who has made six starts for Alaves during his second stint with the club.
The wide man only turned 20 last month and Forlan, who was his manager for a spell at Penarol, insists he remains a long-term project with bags of potential.
"I think he’s adapting really well," Forlan told club media. "When they [United] talked to me about him, I told them everything – he has great potential as a player, good skills, is quick but still young.