The Gunners have just over a week to resolve the issue.
It’s not a problem for Ryan, however, who was thrilled to join the Gunners.
“Signing for the club you grew up supporting as a kid,” Ryan posted on Twitter. “Buzzing to be beginning this new chapter and will give everything I’ve got to contribute to the first club I ever loved.”
On Arsenal’s website, boss Mikel Arteta explained the decision to sign the Australian.
“We know Mat very well through his performances with Brighton in recent seasons and he brings additional quality to our squad,” he said.