"They haven't got a centre forward, have they?" he told Sky Sports.
"He's tried them all, he's brought Ba in there, he's tried going with no striker at Old Trafford - he's tried everything. It just isn't working.
"No matter how many players you've got, your wide player, your No 10s, your floating players, if you haven't got a centre forward a lot of it falls down really.
"All the time I've seen them it's like they are a man down already before they've started the game."