Laurent Koscielny was set to be assessed for a suspected broken jaw suffered in Arsenal's 3-1 FA Cup defeat to Manchester United, boss Unai Emery confirmed.
French defender Koscielny took a boot to the face during an attempted challenge with Romelu Lukaku and had to be replaced by Matteo Guendouzi at Emirates Stadium.
It marked another blow for Emery, who had lost Sokratis Papastathopoulos with an ankle problem in the first half, while Hector Bellerin and Rob Holding are out for the season.
Providing an update on Koscielny and Sokratis at a post-match news conference, Emery said: "I think they weren't big injuries for Sokratis [his ankle].
"Maybe Laurent Koscielny is going to hospital to [analyse] what injury he has. Maybe he's broken [his cheek or jaw] but I hope not."
Arsenal faced an uphill task following quick-fire goals from ex-Gunner Alexis Sanchez and Jesse Lingard and, although Pierre-Emerick Aubameyang pulled one back before half-time, Anthony Martial completed the job to put the Red Devils in the draw for round five.
United were particularly brilliant on the counter-attack and Emery was asked if the balance in his Arsenal side needs remedying.
"Yes, but today for example we worked to stop their counter-attack," he added.
"They have very big players to do the transitions and they gave us problems with that. We worked well, the second goal was in the transition, but the first was [due to] our positioning.
"In the second half, we took some risks and that's what we needed to do. They scored [a third]. We need to keep improving.
"I think the matches against West Ham and Chelsea we played and worked well defensively. Today was more difficult to stop their transition.
"We need to work on attacking [too] because we wanted to win. Our idea was to go in with a good balance.
"I think we have a good balance in a lot of moments but they have big players with big qualities who can make the difference."
Mesut Ozil was again utilised only from the bench, but Emery was uninterested in discussing the reasons for not starting the playmaker, who has been in and out of the starting XI this term.
"He played. I can play other players [too]," he said.
