Strategist Pep Guardiola claims he did everything to persuade Yaya Toure to stay in the team.
The 27-year-old footballer moved to Manchester City at the beginning of July.
"It's a great loss for us," Guardiola said. "I did everything possible to convince Yaya to stay. He was loved by his teammates, but he wanted to leave and I couldn't really stop him.
"I am thankful for everything he has done, and I hope that the club will provide him with the security we couldn't promise - a regular place in the starting lineup."
Speaking about defender Rafael Marquez and forward Zlatan Ibrahimovic, the specialist insisted that they have to make their own decisions about the future.
"Marquez himself has to decide what to do," Guardiola stated. "Ibrahimovic has had a really successful few years. He is a special footballer, so he also has to decide for himself. I haven't talked to the club officials about selling him."