The Frenchman, who managed the Gunners for 22 years, was a pundit on BeIN Sports' panel during City's 2-1 first leg Champions League win against Real Madrid on Wednesday evening.
"I think Man City spent a lot of money on defenders," he said. "If you look over four or five years they spent a huge amount of money.
"That shows that it's not easy to find the defenders who strengthen their team. Everybody is looking for defenders.
"When I was at Arsenal everybody says 'Well you have to find defenders' but we're sitting there saying; 'well, big defenders are not anymore easy to find even with a lot of money'.
"They have disappeared and that's why they have a job to do there. If they lose Laporte until the end of the season it could be a handicap for them in the Champions League. The Premier League is decided so that has gone, now their target they have the quality to win the Champions League but of course they need to be solid defensively."