Bruno Fernandes, Alejandro Garnacho and an own goal from Viktor Kristiansen saw United to victory over Sunday.
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Cooper said, "The result will paint the picture but in the end we didn't show enough desire in either penalty area. We let two unopposed shots from outside the area for the goals.
"Two of the goals have come from our corners as well which is frustrating. It wasn't a dangerous position but we made it dangerous. At the other end we got into good areas but didn't show that same desire.
"Our general play was fine but in the end we didn't have the purpose. We had good spells, we were in the game, it didn't feel like they were going to score the next goal.
"We can't just say we missed Jamie Vardy, we can't be reliant on that."