That's referee Howard Webb over there. A photoshop of him actually, because he doesn't really wear a Manchester United kit while refereeing. He wears a referee's shirt. After a controversial FA Cup match in January, with some controversial decisions going United's way, Liverpool player Ryan Babel posted the photoshopped image to his Twitter. He was fined for it. Basically, everyone thinks Howard Webb is in the bag for their rival. Remember all this, it'll come up later.
Chelsea trails United by three points in the Premier League, but is ahead on goal differential. If they win their clash this coming Sunday, they'll be in the lead with two games remaining. This is a hugely important game for both sides, and one that could effectively decide the league, and with Alex Ferguson complaining about the officiating in last weekend's loss, everyone will be paying attention to the refereeing Sunday.
This is like assigning Danny Crawford to a Mavs game. Oh wait, that happened a couple weeks ago.