Sanchez has failed to make the expected impact at Old Trafford since moving to the club from Arsenal last January.
The Chile international has managed only one assist and no goals in his five appearances for United this season.
Pogba has had a rift with United manager Jose Mourinho and has been strongly linked with the likes of Juventus and Barcelona.
But former Sky Sports pundit Gray believes the duo would have flourished at Anfield.
“There is certainly something missing from Alexis from when he plays for this country or when he played for Arsenal,” Gray told beIN Sports.
“I don’t think that is him alone. When great players aren’t playing at their best they need help as well.
“Pogba isn’t playing at his best, Sanchez isn’t at his best, who is at their best?
“You could probably put Alexis Sanchez in the Liverpool side, if Mane was injured, and I bet he’d play out of his skin.
“The confidence from the Liverpool group would filter through to him. Same with Pogba.
“There is not a collective confidence at United to show what they are made of but I also think the coach needs to allow the players to express themselves in a more open way.”