The Chile international has struggled to make an impact at Old Trafford following a January arrival from Arsenal.
He did record his fourth goal for the club, and first of the 2018-19 campaign, prior to the last international break as he snatched a dramatic last-minute winner against Newcastle.
Sanchez has, however, seen just five minutes of club football since then, having been introduced late on by Jose Mourinho during a thrilling 2-2 draw at Chelsea.
He played no part in a Champions League defeat to Juventus and, as questions continue to be asked of his working relationship with his manager and future in Manchester, the 29-year-old will also sit out a Premier League home date with Everton on Sunday.
Explaining that absence, Mourinho told reporters: “In this moment he’s struggling because he’s injured and we come to the Chelsea match to try to help the team but his physical condition was not good and then he had to stop and was not available for Juventus and is not available for tomorrow again, so now is to be physically in the best conditions.
“He arrived January, in the first period of the season for him, I don’t think it is ever an easy situation for a player but this season he had a good start, a good pre-season, he started well against Leicester, and then he was injured.
“Then he came back and was injured again and he is the kind of player that needs to be really sharp.”
Sanchez is not the only senior figure currently unavailable to Mourinho, with the United boss having once again hit out at international sides for failing to look after his players.
He added: “We play against Newcastle and we go to the national team period in a good situation. Then, after two weeks, we lost [Diogo] Dalot in Portugal, [Marouane] Fellaini in Belgium, we lost Alexis with Chile, we lost who else? [Scott] McTominay with Scotland.
“Some of the national teams were very professional with us and sent the players back and we could start the process early - but some of them wouldn’t and they kept the players and we didn’t even know the dimension and nature of injuries so when the players arrive two days before Chelsea we have no idea of what is going on.
“But we try to solve problems step by step and I think if Jesse [Lingard] can be available and give us some help, McTominay is normally in good condition and we have to wait some more for those other players.”
Lingard has been nursing a groin complaint for some time and has had to be carefully managed by Mourinho.
The United boss added on the England international: “He came from the World Cup with that problem, he thought wrongly that the holiday period would bring things to normality, then he thought wrongly that working would improve it and then he felt in an impossible situation to play football.
“So now he’s going back to do the work to try and be 100 per cent. In this moment he’s almost there because of the team’s needs. Can he help us in the next match? That is the question we have to answer tomorrow or if we wait one more week. He’s almost there.”