The Chilean appeared to pick up the injury in the first half, but looked to have shook it off.
But as he was replaced by Marouane Fellaini in the 70th minute, the 29-year-old held the back of his leg and was thrown an ice pack by the club physio.
Although he hardly set Turin alight with his performance, with fellow striker Romelu Lukaku also struggling with injury, it's a big blow.
But it might even force boss Jose Mourinho to deploy Fellaini to lead the line against Manchester City on Sunday.
The giant Belgian certainly played a huge part in the stirring United comeback that saw Juan Mata level for 1-1, and then Leonardo Bonucci put through his own goal.
On 65 minutes former United hero Cristiano Ronaldo looked to have won it with a stunning volley.
Mourinho first introduced Marcus Rashford on 70 minutes, then nine minutes later on came Mata and Fellaini.
And the substitutions proved a tactical masterstroke by the Portuguese gaffer.
First Mata netted a pearler of a free-kick to level things up in the 85th minute.
Then Fellaini badgered the Juve defence when Ashley Young's cross came in, that resulted in the ball bouncing of Bonucci and in.
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