LONDON (REUTERS) - Manchester United manager Jose Mourinho has questioned his counterpart Juergen Klopp over Liverpool's decision to sign defender Virgil van Dijk on a record fee by reminding him he was once critical of Paul Pogba's world record move to Old Trafford.

Dutchman Van Dijk is expected to join Liverpool from Southampton as the world's most expensive defender with British media reporting the deal to be worth £75 million (S$135 million).

Klopp had spoken about United's transfer spending and their world record signing of Pogba for £89.3 million in 2016, saying he would "do it differently" if he had the same resources at his disposal at Anfield.

"You know, I think the one that speaks about it in a specific way has to be Juergen, and if I was one of you I would ask him about his comments about one year ago," Mourinho told a news conference on Friday.

"The reality is that if they think that the player is the right player for them and they really want the player, they pay his amount or they don't have the player because that is the way the market is at that time.

"Virgil van Dijk is the most expensive defender in history of football, was he better than (Paolo) Maldini, (Giuseppe) Bergomi or (Rio) Ferdinand? You cannot say that. It is just the way the market is and you pay or you don't pay."

Klopp has been surprised by the development in the transfer market. "The last half year changed everything. It's about need and opportunity," he told reporters on Friday.

"If you want to sign a player the last thing I think about is the price - that's not because I throw money around but it's about the player. Not nice but that is the market. We have to adapt. That's how it is."

Second-placed Manchester United are 15 points behind runaway leaders Manchester City and Mourinho said earlier this week his club will need to make bigger investment to be able to produce a team that could challenge for the Premier League title.

Ahead of Saturday's home game against Southampton, Mourinho admitted Romelu Lukaku is running on empty but he could not afford to give his striker a rest.

"I think for a striker, any player, he's absolutely incredible, but if you are a central defender, a holding midfield player, where you can control your energy, positional play, you can resist, survive," Mourinho added.

"But for a striker to play 20 matches in the Premier League, 90 minutes, I have to be grateful."