#1 His desperation to score could work against him

Juventus have been so dominant in the Serie A in recent seasons that they can comfortably win the title without the presence of Ronaldo, and the only reason why the Portuguese was signed was to give the club a shot at ending its poor run in the Champions League.
Ronaldo himself knows this, hence the grief on his face was evident when he received his marching orders against Valencia in his first match of the competition for his new club.
Despite getting off to a good start in the Serie A, scoring five goals in his opening nine matches, it is in the Champions League where he would be desperate to make his mark.
This quest to make a mark for Juventus is bound to be foremost at the back of his mind, and it could work against him, as generally whenever Ronaldo is desperate to score, he ends up with a selfish streak, shooting at will at the expense of teammates even when they are in good positions.
Generally, when Ronaldo is anxious to score, he gets frantic in his play and easily gets frustrated when things don't go according to plan and this desperation could be to the advantage of Manchester United as it could help them shut out their former attacker.