Vincent Kompany, Kyle Walker and Fabian Delph have all cut short their holidays to return to training early on Wednesday at Manchester City, but are unlikely to start in Sunday's Community Shield game against Chelsea.
Kevin De Bruyne, Raheem Sterling and John Stones will be the last players to come back, with their returns on Monday coming less than a week before the Premier League opener at Arsenal.
Belgium midfielder De Bruyne is expected at Wembley on Sunday to watch his teammates, according to his agent Patrick de Koster.
"Kev had permission to stay on holiday until Monday," De Koster told Belgian newspaper HBVL. "He absolutely wants to join the team at Wembley to see them play [against Chelsea]. After the game he will return with the team to Manchester."
Pep Guardiola faces a tough choice for Sunday's game with so many players still returning to full fitness after joining up with the squad on Monday.
Bernardo Silva and Benjamin Mendy were the only World Cup players to join the squad in the United States with senior first team players Aymeric Laporte, Leroy Sane and Mahrez.
It could present an opportunity for Academy players Phil Foden, Lukas Nmecha and Eric Garcia to start the game against Maurizio Sarri's side.
Club-record signing Mahrez was also back in first team training after an ankle injury scare in City's final preseason game against Bayern Munich.
The winger limped off in the 3-2 win over Bayern in Miami and was wearing a protective boot as a precaution when they returned to Manchester.
Ilkay Gundogan is wearing a protective face mask in training after a nose operation on a deviated septum.