Sanchez left Arsenal for United in January 2018 but has struggled to find his best form at Old Trafford, starting just nine Premier League games last season and scoring once.
The 30-year-old has continued to score for Chile, though, netting twice so far at the Copa America, yet Zamorano believes the rust from his club campaign has still been evident in the wins over Japan and Ecuador.
And the former Chile star wants Sanchez, who has been linked with Juventus, to be a starter wherever he begins next term.
"What's best for [Sanchez] is to play - wherever he can, be it at Manchester United or any other team," Zamorano told Omnisport.
"The important thing is for him to be constantly competing. I talk from my experience. Sometimes when you're abroad and you're not playing, it is hard to regain the level.
"We've seen it. Alexis hasn't played in Manchester for a long time, but the second he puts on the Chilean shirt, he's different. That's what we like.
"No matter how you are doing at your team, we hope at the moment you put on the Chilean shirt you give your best, and that's what Alexis is doing.
"I'm very happy to see him regaining his level and to see him being a vital part of Chile, being a protagonist in South America again."
Sanchez was set to be assessed for an ankle injury ahead of Monday's clash against Uruguay, with Chile having already sealed their progression out of Group C.
Man Utd have already brought in Daniel James from Swansea after the 21-year-old impressed with his pace in wide areas for the Championship side last season.
James will provide extra competition for Sanchez next term with Ole Gunnar Solskjaer said to be keen to reshape his squad with young players.
Senior players Romelu Lukaku and Paul Pogba could to depart Old Trafford this summer, with Ander Herrera having already departed following the expiration of his contract.
James could get his first competitive opportunity for the Red Devils in their opening Premier League game against Chelsea on August 11.