Graeme Souness believes Manchester United's spending on defenders "will all be money down the drain" if they are unable to improve their midfield.
United forked out £125million on Harry Maguire and Aaron Wan-Bissaka in the summer, with Victor Lindelof and David de Gea also signing new and improved deals since the start of the season.
But Souness, writing in the Sunday Times, believes United's back-five will be "fire-fighting" until they improve in their midfield.
"Until they fix it (their midfield), United's back five will be fire-fighting," Souness wrote.
"They can spend massively on Maguire, give Victor Lindelof a new five-year deal, De Gea four more lucrative years and sign Wan-Bissaka for £50million and it will all be money down the drain.
"United are trying to rebuild from the back after signing Harry Maguire from Leicester City for £80million and Aaron Wan-Bissaka from Crystal Palace for £50million.
"I rate Maguire. He will be disappointed with how it's gone so far, but I'd not put it all at his door'.
Souness continued: "When a team is conceding goals, people focus on the goalkeeper and back four. Their mistakes, their vulnerability, how they seem to be caught out of position.
"But most of the time that's because the midfield in front of them are not doing their job properly, so they have to step out and confront situations or make saves.
"Their midfielders were under the illusion they were doing their jobs because they were goal side, but when somebody in 20 yards from goal you can't allow them to get their head up as West Ham's midfielders did before Andriy Yarmolenko scored.
"United's midfield are too easy to play against. Nemanja Matic, Scott McTominay and co should have been sprinting out to put West Ham's midfielders under pressure."
