Liverpool defender Virgil van Dijk will be out for "more than a month" with a hamstring injury as he suffers a fresh injury blow, manager Jurgen Klopp said on Friday.

Van Dijk sustained the injury in Liverpool's 3-1 defeat to Brentford on Monday and was substituted at half-time.

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"Virgil was a surprise to us. It was a big blow," Klopp added in a news conference on Friday.

"He didn't feel a lot. I took him off, it was not clear that maybe he would have tried [to play on]. The diagnosis was pretty harsh. We talk about weeks, more than a month but I hope it goes quick. I hope it goes quick, we'll have to see. For now he's not available, that's how it is and we have other centre-halves.

"Everything is okay for the team but for Virgil it's hard for him. He's played an incredible amount of games over the years. We cannot use him on the pitch but off the pitch we will do that."

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Meanwhile, Liverpool will enjoy the addition of new signing Cody Gakpo, and Klopp said he has been impressed with the winger in his first days of training.

He's a good footballer, good in small spaces, likes to shoot from distance, good finisher and dribbler," Klopp said.

"Everybody who wants to know about him could know as there are a lot of videos out there about him. It is nice to see in real life, in front of you.

"He looks really promising and fit. The sessions are intense for him because they're different. I'm very positive about him. Coming from a new league is always difficult, but we expect a positive impact."

Liverpool are in sixth place in the Premier League, seven points behind fourth-place Manchester United.

Klopp's side host Wolves in the FA Cup third round on Saturday before a trip to Brighton in the Premier League next weekend.

"There have been easier moments to sign for Liverpool," Klopp said.

"This year we can't guarantee Champions League football, but Cody never asked. He knew the decision. He can see the table. He's a guy who doesn't want to jump on a running train, he wants to push the train, that always helps."