"We can produce these exciting games okay, but I can't understand our defending," Ferguson is quoted as saying by The Telegraph. "It's making it difficult for us. I can't get to the bottom of it, I'm afraid.
"If you analyse the goals we are losing they are all different types. Players were left free in the box tonight.
"We are not having a good start in games, that's for sure. It's interesting it's in the home games we are losing the early goals, but we will sort it. I'm sure of that."
United are currently at the top of Group H with three wins from three matches.