Arsenal fear Cristhian Mosquera will miss at least six weeks to add to their defensive problems.
The Spanish defender is set for further tests to establish the full extent of the ankle injury suffered in the 2-0 midweek win over Brentford.
Mosquera's absence will be another blow to the Gunners, who also currently have first-choice centre-back pairing Gabriel Magalhaes and William Saliba sidelined.
The 21-year-old required treatment after landing awkwardly before being substituted, and early concerns are he will miss between six and eight weeks.
Arsenal manager Mikel Arteta said his summer signing from Valencia required further examination as he added: "Mosquera is the one that is more complicated. We have another test today."