City have been shipping goals since the Irishman's departure, a big error according to Hamann.
"I couldn't understand why they let him go," Hamann told the Daily Express. "He is a top-class defender and held the team together. Now they look vulnerable at the back.
"Hughes has bought well, although he's paid too much for certain players, and has assembled a big squad. But there are already suggestions some are not happy because they are not playing week-in, week-out.
"When things don't go well we will see how they handle the pressure and if they have the spirit to get the team out of trouble."