Jamie Carragher believes Lisandro Martinez is 'too small' to be an effective central defender for Manchester United and could be a liability at set-pieces.
The Argentine World Cup winner won plenty of plaudits for his tenacity during his first season at Old Trafford before being struck with a metatarsal injury which led to two spells on the sidelines.
Martinez has only just returned to action for Erik ten Hag's side following the second of those lay-offs, coming off the bench in the Premier League draw with Tottenham and then starting the FA Cup win at Newport County.
Carragher isn't convinced Martinez, who stands 5ft 9in tall, is adequate for United to be watertight from corners and free-kicks.
'I said this when Manchester United signed him, but I did worry about Lisandro Martinez for his height,' Carragher told the Stick to Football podcast, brought to you by Sky Bet.
'If I was a manager, I would never buy a defender that small and the reason being it's not just him, I like him as a player and he's got great spirit, he's brilliant on the ball, he's aggressive.
'But the way football is now, probably 20 years ago I used to play full-back, and you'd normally have a back four, one of your full-backs would be tall as well for set-pieces.
'But the way the games now, because full-backs are almost like wingers, there's not that many big players in your team, so your full-backs aren't the biggest anymore.
'I always think for set-pieces, your centre-backs sometimes and even your striker now - how many teams play with a massive player up front - it doesn't really happen as much.
'Your two centre-backs have got to be, just for set-pieces, 6ft 3in, 6ft 4in, so at least you've got those two pillars.'
Ten Hag knew all about Martinez from his time coaching Ajax and signed the 26-year-old in a deal worth an initial £47million, plus a potential £8.5m in add-ons, in July 2022.
Martinez quickly allayed fears over his height during an impressive first season for United in which he played 45 times and helped them win the Carabao Cup.
But he suffered a metatarsal fracture in his right foot playing in the Europa League quarter-final against Sevilla on April 13 and missed the remainder of the campaign.
Martinez featured in the opening six matches of this season before aggravating the foot problem in the Champions League match against Bayern Munich in September and requiring surgery.
The Argentine is likely to feature when United make the trip to Wolves on Thursday night.