Mark Lawrenson feels Manchester City's transfer dealing in the summer is the "first glaring mistake" Pep Guardiola has made as manager.
City chose not to bolster their defence despite the departure of captain Vincent Kompany in the summer.
Between injuries and poor form for some players, City have been counting the cost.
The downturn in performance this season was compounded in the 3-1 defeat to Liverpool on Sunday which leaves Guardiola's side nine points adrift of the Premier League leaders in fourth.
Speaking on Off The Ball, the ex-Liverpool and Ireland defender expressed surprise at the decision not to strengthen.
"This is the first glaring mistake Pep Guardiola has made," he said.
"He had the whole summer. [Harry] Maguire obviously, who's a very good player not a top, top player but you're not telling me he couldn't have improved what they've got at the back at the moment.
"Otamendi has always been a player who's made many, many mistakes. John Stones has never really kicked on. We knew about his footballing ability but at Everton and he looked very, very good.
"The problem is when push comes to shove, he makes terrible defensive decisions. But to sit every day in the office in the summer or wherever he was, Guardiola, and say, 'I'm not going to buy a centre-back, not going to buy a centre back...' If you're a fabulously wealthy team, you just go and buy one."
Lawrenson added that even extra competition of a back-up could have a benefit on the defenders already at the club.
"They turn around and go, 'Wow, he's come to play in my position', and then you improve your own performances. So it's a really, really strange one."