Bilic admitted the root of the Hammers' issues at Old Trafford in the 4-1 defeat were easy to pinpoint as they conceded inside the opening three minutes in either half either side of an equaliser from Ashley Fletcher, a former United academy prospect.
"I'm very disappointed, they deserved it," said Bilic afterwards.
"You can't have a slow start anywhere, especially not here because they're good anyway but when they're one up they hit on the counter attack, have amazing quality and are very direct and punish you.
"They could've scored a second one after the first but we defended and then came back into it.
"At half-time we said we couldn't allow them to do it again but it was exactly the same start in the second half."
He added: "It's hard to come back twice and then they scored the third one and that's them totally in control of the game. You can't expect anything here when you have such a slow start to the game."