Per Mertesacker has been ruled out of Arsenal's United States tour after suffering a knee injury.
The Gunners' most experienced centre-half played in Friday's friendly at Lens and was named in the provisional squad to travel to the US this week.
But injury means he will not be among 23 players on tour, leaving Calum Chambers as the senior central defender in the absence of Gabriel and Laurent Koscielny.
Gabriel has tonsillitis, while Koscielny will miss the start of the Premier League season after helping France finish as runners-up at Euro 2016.
An Arsenal statement said Mertesacker would remain at the club's London Colney base but did not go into detail about the severity of his injury.
New signing Rob Holding could partner England U21 colleague Chambers in the States, while Nacho Monreal, Mathieu Debuchy and young Pole Krystian Bielik all have some experience at centre-half.
Arsenal play an MLS All-Star team on Friday and finish their tour against Chivas on Monday, and Granit Xhaka will make the trip with his new team-mates.
But Alexis Sanchez, Aaron Ramsey and Mesut Ozil are among the big names left out following their summer international commitments.
Sanchez is also recovering from an ankle injury suffered during Chile's Copa America final, although Arsene Wenger said at the weekend the problem is "not too bad".
Full squad: Cech, Ospina, Martinez, Debuchy, Monreal, Holding, Bellerin, Chambers, Gibbs, Coquelin, Bielik, Elneny, Wilshere, Iwobi, Xhaka, Cazorla, Zelalem, Reine-Adelaide, Oxlade-Chamberlain, Walcott, Willock, Campbell, Akpom.