The club added in a statement: “Everyone at Manchester City wishes Pep a speedy recovery, and look forward to seeing him back in Manchester soon.”
Lillo, 57, spent two years at City between 2020 and 2022, leaving the side last summer to take up a head coaching role at Qatari side Al Sadd.
City had been working to bring Lillo back to the Etihad this summer after Enzo Maresca left Guardiola’s coaching team for the top job at Championship side Leicester City.
Lillo’s coaching career began in the lower divisions of Spanish football but he has had roles across the globe, including in Japan, China and Colombia. He also managed Guardiola towards the end of the former midfielder’s playing career, during their time together at Mexican side Dorados de Sinaloa.
Following the game at Bramall Lane, City host Fulham on September 2 before the international break between September 4 and 12.
City then travel to West Ham on September 16.
Analysis by Sam Lee
What Pep Guardiola has gained with Lillo’s return and lost with Maresca, Borrell exits
Juanma Lillo only left last summer to take up a coaching role at Qatari side Al Saad, but his return ahead of the season was vital because he has such a close bond with Guardiola — and has already made a big contribution.
Guardiola believes Lillo helps him be more calm and analytical, and keep his emotions — which often run high — in check.
Lillo has impressed with his individual work with players, despite not learning English to any great degree.
His presence has also helped Guardiola maintain harmony in the City dressing room — notably when morale was affected by strict Covid-19 restrictions at the club.
Guardiola said earlier this month when Lillo returned: added: “Juanma sees things no one else in the game sees. He understands football on an incredible level, so he is the perfect person for me to work alongside.
“I have always been inspired by him — his knowledge of football, his intelligence and his humanity mark him out as a very special person in my life — and we have a shared ideology.”
(Photo: Claudio Villa/Getty Images)