Alan Pardew thinks new signing Victor Lindelof is a concern for Manchester United after his performance in the 2-1 defeat to Real Madrid in the UEFA Super Cup on Tuesday.
Lindelof joined United from Benfica for an initial £30.7m (€35m) in a deal which could rise a further £8.8m (€10m) based on performance incentives but has struggled so far in pre-season, putting in some less than impressive displays.
The Sweden international gave away a penalty in United's victory over Real Madrid last month, clumsily clattering into Theo Hernandez and he was at fault again in the UEFA Super Cup, losing Casemiro as the Brazilian opened the scoring.
And new Sky Sports pundit Pardew, speaking on The Debate - our new nightly show on Sky Sports Premier League - is worried by what he's seen of the 22-year-old so far.
"I thought United's signings were particularly good with Romelu Lukaku at the top of the pitch, Nemanja Matic in that midfield role and Lindelof at the back," the former Crystal Palace manager said.
"They needed a centre-back in particular but Lindelof's performance against Real Madrid was concerning and there's been a bit of concern over him in pre-season.
"They've had injuries in that area," he added. "Smalling never seems to play a full season and Jones is always injured so there's going to be huge pressure on Lindelof.
"At the moment I think he's finding it a struggle. Benfica and Portuguese league is not the challenge the Premier League is week in, week out. The intensity of set plays and the pressure put on central defenders is very, very difficult.
"I always remember Sir Alex Ferguson saying to me that at United, because you are Manchester United, your two centre-backs have to deal with two-on-two situations. You have to deal with one-on-one situations because you commit all your players forward to try and win the game with the opposition defending so deep. Lindelof's one-on-one play worries me."
United, who had the second best defensive record in the Premier League behind Tottenham last season, start the new campaign against West Ham on Super Sunday - live on Sky Sports Premier League.
And Pardew thinks Lindelof will have to adapt to life at Manchester United quickly if he's going to improve a defence that conceded only 29 goals last season.
"He didn't put any personality on the pitch [against Real Madrid]," Pardew said.
"If you are a Manchester United player you've got to have personality, you've got to have that aura about you.
"Of course it's only his third or fourth game and he might grow, but he's going to have to grow quickly.
"And hopefully for Manchester United's sake he does because he will start on Sunday. He's going to have to step up to the plate quickly."
Watch The Debate every weeknight at 10pm on Sky Sports Premier League. Wednesday's show features Craig Bellamy and Phil Neville.