The 30-year-old has been a shadow of the player who excelled at Arsenal since making his move to Old Trafford last January in a swap deal with Henrikh Mkhitaryan.
The former Barcelona star has scored just one Premier League goal in 13 appearances this season and former Liverpool defender Carragher says it reminds him of Torres, who flopped at Stamford Bridge after making a then-British record £50 million move from the Reds in 2011.
"Under Mourinho he (Sanchez) would consistently lose possession," Carragher said on Sky Sports.
"He's got to do more himself. Looking at him in a Manchester United shirt was like looking at Fernando Torres in a Chelsea shirt.
"You see this blistering pace and power in one of the stars of the Premier League - but it just seemed to go.
"Does that go through a lack of confidence, the manager’s instructions, or the amount of football the lad has played?
"What he does for his country has maybe played a part. He has to go so far over there for games. And he’s consistently played football for a while now."
Neville suggested that age might have caught up with the forward, who turned 30 in December.
“He looks like he’s lost a little bit of his spark and his pace," he said.
"If that is the case, can he adapt his game?
"Can he get in behind still, face a man up, or become more of an assist player, a hold-up player? When a player loses that little bit of pace, he starts to drop in and move into areas where it’s easier to pick the ball up.
"He looked to me like a player who is doing the simple thing - drop, beat players with his back to the goal, and then lay it off, rather than turning directly on people.”
