Chelsea No1 Cech: Van Persie has improved Man Utd
Cech says the signing of Van Persie has made United even more of a threat.
The goalkeeper, 30, explained: "He's added goals for United and this is the most important thing.
"He is somebody who can score in every game and it is clear he is another attacking option.
"With Rooney they can be a very interesting pair and you can see they have been scoring. So we'll see. It will be another exciting game but we must defend better than in Donetsk.
"On the other hand, they've been leaking goals so it will be exciting if both teams play like this. It could be entertaining."