Chelsea need someone that can have the same impact on their effectiveness at set-pieces as Virgil van Dijk did at Liverpool, says Frank Lampard.

The Blues twice saw Tomas Soucek put the ball in the net from corners during a 3-2 loss to West Ham on Wednesday, though his first was disallowed due to a contentious offside decision following a VAR review.

Lampard acknowledges dead-ball situations represent one of the Blues' biggest weaknesses and feels it is something they should take into account when assessing transfer targets.

The Chelsea head coach pointed to Liverpool's improvement from set-pieces after they signed Van Dijk, a figure who has been pivotal in their ascent to becoming Premier League champions.

"We've looked at set-plays this season. When we came into the club, we looked at the stats from last season and we were bottom of the league in defending set-plays, in terms of other teams getting big chances against us, we were second-bottom in attacking," said Lampard ahead of Saturday's Premier League meeting with Watford.

"If you look at personnel, and I don't like to talk about my playing days, but we didn't really work so much on set-pieces at all really - with certain managers more than others. There were probably a few reasons why and they were mainly John Terry, Gary Cahill, Didier Drogba, Michael Ballack and [Branislav] Ivanovic, so you stick it in an area and they defend it or they score goals.

"I think similarly with Liverpool, there was a lot of talk about them with set-pieces two or three seasons ago, they were zonal, they were conceding a lot.

"They signed Virgil van Dijk and he heads everything that comes in the box out, so there's a huge relation to personnel and if you don't have that in terms of big size across the team, I think trying to compete as hard as you can, making it difficult for other teams to score is the main thing.

"And there's no doubt – I'm not making excuses when I say that because we've not done this well enough on the pitch, and that has to improve – when you look at who you might bring in, with the balance of the squad, in the modern day its certainly something to take into account when looking at recruitment."

Chelsea made Kepa Arrizabalaga their record signing and the most expensive goalkeeper in the world by paying the £71.6million (€80m) release clause in his Athletic Bilbao contract in 2018.

Lampard dropped Kepa for Willy Caballero earlier this season and reports of interest from Spain intensified after the 25-year-old put in another disappointing display against West Ham.

Asked about the rumours linking the keeper with a move away, the Blues boss said: "I'm not talking about any potential [deals], what we may do at the end of the season or not. I'm going to focus on what we've got in front of us.

"With Kepa, there's been a lot spoken about him this season. All I want from him, as with every player, is the right mentality and attitude and approach to the game – I'm absolutely certain I'm going to get that from all of them because I can feel the group is in that position – and obviously to just try to do his job as well as he can do.

"I'm happy with him and that's how we keep working. I'm not going to talk about what may be at the end of the season."