The easy explanation of Manchester United's dramatic-fashioned equaliser against Chelsea on Sunday is to blame it on Ivanovic's dismissal. Despite the second booking obviously not was being irrelevant, José Mourinho didn't help himself through applying a zonal marking approach on set pieces.
As we see on the picture, the Chelsea players have lined up in a zonal marking set-up. They were almost punished by this in the first half, when Angel Di Maria cut back to the unmarked Juan Mata, who skied it, so Mikel covers up Blind and Shaw, but they still don't fancy the man-marking approach. (I'm aware that they practice on this every day and changing it mid-game could/would cause unnecessary confusion, but this doesn't change my opinion on defending set-pieces through zonal marking.)
Attackers gain momentum
This sort of tactics allow the attackers to run onto the cross, getting on the front foot, and that makes it extra difficult to pick up a tower like Fellaini. If applying a man-marking set-up, the defender, no matter how small or tall he is, can approach bodily contact, maybe tug a little bit in the shirt, give a slight push when the attacker is about to jump and generally block his path to the cross. Zonal marking gives the attacker an opportunity to get gather some pace before the eventual bodily contact occurs, which makes him way harder to pick up.
United aerial underdogs
Considering the fact that Chelsea had more aerial artillery than United, both before and after Ivanovic got dismissed, they would've had a decent chance to cancel out United's threat in that department. The Red Devils had a single major threat in the box - Fellaini - and Matic, Drogba or Terry would've probably cancelled him out. They even would've been favourites against Jones and Van Persie, considered United's second and third best headers. Additionally, Kurt Zouma is probably a fair shout in the air (I haven't watched him, but he's a centre back after all), meaning he could pick up somebody like Rojo. And, for other set-pieces bar this particular one, they had the tank Ivanovic as well. In other words, Mourinho's love for having aerially strong players would've put them in the pole position by some distance against the likes of Wilson/Mata, Januzaj, Rafael and Blind.
However, they weren't able to benefit from this, because of the zonal marking. They gave the likes of Van Persie and Fellaini the opportunity to run onto the cross, failed to pick up the latter who won the aerial challenge, and the former blasted home the equaliser.
Obviously I, as a United supporter, celebrated as much as anyone, but this once again fuels my contempt towards defending this way.
Zonal marking wouldn't have helped United either
You might very well table the argument of "but United conceded through man-marking" at this point. Well, that's true, but that's because Chelsea were aerially dominant and Rafael had to mark Drogba. Chelsea simply had too many top class players for United to cope with aerially. Particularly considering Van Persie doesn't man-mark, as he's tailored for the 'zonal-mark the near post and drift towards the goalline for a clearance if the ball doesn't hit you' role. That was inches away from paying off, as he got his head on Drogba's opener, despite being unable to get it away because of the close range and power. In other words, zonal marking all over the place and letting Matic, Ivanovic, Drogba and Terry run at you for the entire game hardly woud've improved United's chances on set-plays anyway.