Sanchez joined United from Arsenal last January in a deal which saw Henrik Mkhitaryan go the other way.
The Chile international – who is on a staggering £390,000 a week – has endured a torrid time at Old Trafford, scoring four goals in 30 appearances for the Red Devils.
The former Barcelona man has looked a shadow of the player who lit up the Premier League during his time with Arsenal, in which he bagged 80 goals and 45 assists in three-and-a-half seasons.
And Winterburn believes the 29-year-old’s poor form is down to him not being suited to the style of football United boss Jose Mourinho plays.
“I didn’t think Alexis Sanchez’s style of play would suit United, he’s a free spirit and you have to know how to deal with him,” he said.
“He’s got undoubted ability, but I wasn’t quite sure the way he plays would fit in to the Mourinho way.
“I look back at Sanchez for Arsenal, and there’s something not quite right with him at United, and it will be interesting to see how the situation will develop.
“It will be interesting if he stays and plays a major part for the club or whether he looks for a new challenge, but he’s not hit the level of performance from his time at Arsenal.”
Sanchez is currently on the sidelines with a torn hamstring, and Mourinho is expecting the Chilean to be unavailable for the foreseeable future.
“There have been no scans yet but the player is experienced, the player knows what the injury is and also from the top of my experience, just the painful scream and the way the injury happened, I know it’s going to be for a long time,” the Portuguese said of Sanchez’s injury last Friday.
“It is not the little muscular injury that in one week or 10 days the player is ready.
“It is, for me and also for Alexis, the feelings before the scans is that the grade [of tear] is different than the Lindelof one and the Lindelof one was not an easy one so Alexis has what I used to call an aggressive muscular injury.
“He has an important injury. From the top of my experience, he is not playing this year.”
Sanchez’s hamstring injury has ruled him out of tonight’s match against his old club, who travel to Old Trafford unbeaten in their last 19 matches.
Mourinho is anticipating a tough match against the in-form Gunners, who saw off their bitter rivals Tottenham last Sunday.
“It’s a very difficult match for us,” admitted Mourinho. “We play against a good team in a good moment with a good run of results [but] without the accumulation of matches because they play Europa.
“When you play Europa League you normally play with second choices and you can give a good rest to your [first team] players compared to those in the Champions League, where we have to play in the maximum of our potential. It’s an accumulation of matches and you arrive in December where many teams pay the price for that.
“When you play Europa – I have done that – it’s not the motivation for the club but it’s good to develop the team and to have people fresh.
“Plus the confidence of the good results because when you play Europa you have good results because you win easily.
“Very difficult match for us but very difficult match for them in spite of our problems, with players, with selections in spite of that it won’t be easy.”