City said their treble-winning manager had been suffering from "severe back pain for some time" and flew out to Barcelona for "emergency" yet "routine" surgery.
Manchester City signed Croatia center-back Joško Gvardiol from Leipzig for 90 million euros ($99.2 million) on Saturday, ranking him among the most expensive defenders in soccer history.
Schweinsteiger, who works as a TV pundit, played under Guardiola for two seasons at Bayern following the former Barcelona coach’s arrival in Munich in 2013.
Enzo Maresca became part of Guardiola's staff in 2022, helping City to the greatest season in its history, including winning the Champions League title last weekend.
This was the final frontier for Guardiola's team, which also completed a rare treble of trophies after winning the Premier League title and FA Cup this season.