Well, so much has already been said, might as well add 2c:
1. Horrific injury, real shame for it to happen to such a promising young player. Hopefully it'll heal and he'll come back soon. I don't think anyone disagrees on that front.
2. The foul itself was reckless, regardless of intent. It was too risky for the potential gains, in my opinion, and warranted a yellow. That said, given the result, I can totally see the reason Walton gave a red. But in theory decisions should be based on the action, not the result.
3. Arsenal fans are more than welcome to wrap themselves in the comfy blanket that they play football the way it was meant to be, and all other teams are simply talentless hard men that in their sheer envy of one-touch keep ball feel they must kick Arsenal to the curb. But it sounds a lot to me like Liverpool fans claiming that the only reason United have ever won a trophy was due to the league/refereeing/god being corrupt. It's just not true.
4. This whole '3 makes it more than a coincidence' thing is frankly insulting. You're taking 3 incidents over the course of roughly 5 years and however many tens of thousands of minutes of football to demonstrate a trend? Seriously? So is 2 coincidence and 3 not? Who decided that? How far back do we look? Do we include partial breaks? No. It was an isolated incident.
5. Wenger was pissed the hell off, distressed and in a way it's like watching a son get hurt. So cut him all the slack he needs on his comments, if he reacted rationally there'd be something wrong with him.
6. If Arsenal win the league (when?) it'll be a great victory for football, and a really great occasion. And I'm a United fan.