Sanchez has been strongly linked with a loan move to Inter Milan this transfer window, and ex-England fullback Neville suggested a move couldn't come quickly enough for the red devils.
“I welcomed [Sanchez] coming to the club as he’s a tenacious forward that plays across the line and scores goals," Neville told Sky Sports’ Monday Night Football.
"But it’s been a disaster. I have no idea what's happened.
"There must be two of him. The one that played for Arsenal and Barcelona and then the one that’s turned up in Manchester.
"They need to get him out of the club."
Sanchez established himself as one of the best players in the Premier League in a three-and-a-half year spell with Arsenal.
The Chilean attacker moved to Manchester United in January 2018, signing for wages reportedly over £500,000 (A$890,000) per week - making him the highest-paid player in the league.
However he failed to rediscover his form in Manchester, managing just five goals in 45 matches.