was injured in a freak incident during Manchester United
's defeat by Paris Saint-Germain
- colliding with a linesman while he was warming up.
Chile star Sanchez is now battling to be fit for Monday's FA Cup tie against Chelsea at Stamford Bridge, with Anthony Martial
and Jesse Lingard already serious doubts for the fifth-round clash.
Sanchez was preparing to come on for Lingard, who had strained his left hamstring towards the end of the first half in the 2-0 Champions League defeat.
However, the United substitute became distracted by the match action as he jogged along the touchline and ran straight into Italian linesman Fabiano Preti.
The collision floored Sanchez who recovered sufficiently to replace Lingard moments before half-time, but he informed United's coaching staff that he had suffered a leg injury above the knee.
The decision was taken to keep him out on the pitch to stop his leg from stiffening up and Sanchez spent the interval warming up in a training top.
The bizarre incident is the latest setback in Sanchez's troubled United career since he arrived from Arsenal 13 months ago as the club's highest earner on £500,000 a week.
The 30-year-old was recovering from a hamstring injury when Ole Gunnar Solskjaer replaced Jose Mourinho in December and has started only three times in 12 games under the new caretaker manager. He has scored twice this season.
Sanchez once again failed to impress off the bench against PSG and Solskjaer admitted that only the player himself can solve his problems at Old Trafford. 'I'm not the one, I can't do anything about Alexis Sanchez,' said United's manager. 'When he plays, he needs to find himself because we know there's a quality player there.'
While Solskjaer returned to Norway to see his family and the squad were given a day off, United were anxiously waiting for news on Sanchez, Martial and Lingard on Wednesday. Martial was replaced by Juan Mata at half-time after appearing to aggravate a groin injury.
The Frenchman started for the second time in four days on Tuesday night with Solskjaer seemingly reluctant to name Sanchez in the line-up against either Fulham or PSG.
A miserable night for Solskjaer against PSG was compounded when Paul Pogba was sent off towards the end of the game. He will miss the second leg in Paris early next month.
However, Ander Herrera believes United's away wins over Tottenham and Arsenal under Solskjaer gives them hope that the last-16 tie can be salvaged.
'We have good examples,' said Herrera. 'We won away against Tottenham, we won against Arsenal.
'They are teams that are as good as PSG, so we will go there to try and make it difficult for them.'