The Chilean became the Premier League's top earner on £600,000 per week when he joined the Red Devils from Arsenal, but he has scored just once in 10 games since the switch.
According to the Daily Mail, Sanchez is struggling to settle at Old Trafford and is often seen eating alone at the club's canteen.
The United hierarchy has allegedly expressed concerns that the forward could go the way of Angel Di Maria, who joined the North-West outfit in a £59.7m deal only to be shipped off to Paris Saint-Germain for £44m following an underwhelming debut season.
Red Devils chiefs are also said to mindful about the possibility of Sanchez's hefty wages causing a rift among the United squad.
Manager Jose Mourinho, meanwhile, is reportedly confident of getting the best out of the former Barcelona star next season.