It really would be the worst and most devastating feeling to lose him. Much much worse than Fabregas. For years we always knew Fabregas was eventually going back to Barcelona, and while it cause a lot a of anger in the way it happened, by the time it was all over I was glad to see the back of him and hope I never have to hear of him again.
But with Van Persie it would be much different. This is a guy who is Arsenal through and through. Grew up a fan dreaming to play for us. We went through all those years, season after season of him being injured for at least half a season, often much longer, and finally he has 1 season where he doesn't get injured and is the by far the best player in the league and not too far off even the best player in the world. Would be extremely hard to pick ourselves up from losing this guy. He simply has to be kept.
