Arsenal lined up in a Premier League game without either Gabriel or William Saliba for just the second time in 162 games on Sunday after an injury to the Frenchman prior to match at Chelsea.
With Gabriel already ruled out until the end of December with a thigh injury, there was surprise when the team sheets were announced as Saliba, Arsenal's next most senior centre-back, was absent.
Piero Hincapié was partnered by youngster Cristhian Mosquera at the back.
Arsenal said Saliba sustained a "knock in training," while manager Mikel Arteta said ahead of kickoff: "Yesterday in training he was uncomfortable, so we need to explore a little bit more what happened.
"I don't think it's something major, but enough not to be involved in the game."
The only other time Gabriel and Saliba had been missing in the Premier League since both were at the club was on the final day of last season, a 2-1 win at Southampton.
- Follow Chelsea vs Arsenal live with ESPN
Saliba had played the full 90 minutes in the win against Bayern Munich in midweek.
His knock doesn't come at an ideal time for Arteta, with a demanding fixture list upcoming. Arsenal play nine games in the next 39 games, starting with a clash against Brentford on Wednesday.