In his pre-match press conference, Arsene Wenger reported that Laurent Koscielny and Petr Cech were 80% likely to be in the squad for Monday’s match against Middlesbrough. Both were joining in training today and were going to have tests, with Wenger fairly certain that both would be available.
Koscielny and Cech are certainly better than their replacements, and had Arsenal had them available, maybe one of the three goals conceded against Crystal Palace would not have happened. But Arsenal are not a good defensive side even with Koscielny and Cech, and so while their return is welcome, Arsenal also need to defend better as a team. Wenger said as much in the press conference, but Wenger has also been saying that for weeks without the team actually improving.
As for other injuries, David Ospina and Lucas Perez remain injured. Ospina has a back injury, Perez has an unspecified leg injury, which has suspiciously kept him out for weeks.