Juventus coach Massimiliano Allegri believes Marouane Fellaini made the difference and helped Manchester United to a 2-1 win in the Champions League.

Fellaini was introduced with United trailing to Cristiano Ronaldo's stunning volley and the visitors claimed a surprise win thanks to Juan Mata's free-kick and an Alex Sandro own goal on Wednesday.

The Belgium international almost got on the end of Ashley Young's set-piece that led to the winning goal and Allegri said Fellaini was decisive in the Group H clash.

"In the first game, they didn't have Fellaini and we used his absence to our advantage," Allegri told a news conference.

"Tonight, he played and his physical presence was immediately felt. He controlled some high balls, although sometimes we were good at playing the ball on the ground.

"Conceding those free-kicks was our greatest mistake. That happened and we paid the price.

"We'll have to learn from that."

Juve remain top of Group H on nine points, two clear of United and well ahead of Valencia (five) and Young Boys (one).