United won 3-2 thanks to an injury-time VAR penalty decision, which saw Bruno Fernandes convert for the winner.
“I think the first half was good from our side - we created a lot of big chances, I think we didn’t concede that much,” the Dutchman told MUTV.
“Overall a really stable and solid performance but obviously you keep them in the game with not scoring the 2-0 or even 3-0 - that’s something we could improve.
“But also in the second half, you felt like we dropped in energy and retention and then you see out of nowhere they could score goals and make it difficult for us.
“That’s something we definitely have to improve, but I think the fact we came after the first goal of theirs - immediately we scored 2-1 and after 2-2 we still believed to score 3-2 - I think is the biggest plus of today, but we all know that getting away like today is not every week so we really have to improve on that.”