The Reds shipped another three goals in last night's draw at Valerenga.
Kuyt said: "We are letting in too many goals but the point is we have to defend like a team.
"That starts from the front and we need to improve. We know we have the ability and we just need to train really hard."
He added: "I'm not concerned as it's still pre-season. If we beat them (Sunderland) in our first game then it's been a good pre-season for us."