I don't think Cooke's hit was dirty, and there was no intent on his part to injure. It looked like Cooke was trying to make a hit and planted his leg. Unfortunately, his knee was sticking out, and it hit Donovan's knee squarely. It was a really unfortunate incident, and it's not the first time we've seen something like this.
The injury looked really bad from vantage point at the Bank. Donovan couldn't put any weight on his leg and was grimacing in pain. Neil came off the bench to aid him off the ice, and you could see Donovan wouldn't have made it to the bench without Neil's help.
I hope this doesn't end his career. But given his age and that his biggest asset is his speed, this could very well be his last hoorah. I hope not, but it's possible.