Ajer returned to action in an away defeat against Chelsea earlier this month, featuring as a substitute after missing seven Premier League games with a foot injury.
The Norway international was then given his ninth Premier League start of the season in Saturday's home clash against Nottingham Forest.
Frank's side experienced a disappointing outing at the Gtech Community Stadium, with Ola Aina and Anthony Elanga grabbing a goal apiece to condemn Brentford to a 2-0 defeat, bringing an end to their unbeaten home record.
Unfortunately for the Bees, their afternoon was made even worse by the fact that Ajer was forced off with an injury issue in the second period.
Frank confirms Ajer injury setback
Frank has now revealed that the Norwegian is set for another spell on the sidelines after sustaining an ankle injury against Forest.
The Brentford boss has also confirmed that Ethan Pinnock and Sepp van den Berg remain sidelined with hamstring and groin issues respectively.
"On the injury situation, Ethan [Pinnock] is the same as last week: it's a bigger one, it'll be weeks with a hamstring injury," Frank told reporters ahead of Friday's clash with Brighton.
"Van den Berg is minor but it's going to be too short a turnaround for Brighton... let's see for Arsenal."
"Ajer went off injured and, unfortunately, picked up an ankle injury. It's a similar one to the Yoane Wissa earlier this season and Pinnock's last season. [Ajer] will also be out for weeks."
Will Jensen be ready for South-coast trip
Jensen has missed Brentford's last five competitive matches after he picked up a hamstring injury in last month's 4-1 win over Leicester City.
The Denmark international is closing in on a return to fitness, but he will not be ready for Friday's trip to the Amex Stadium.
"Mathias Jensen is positive, he's getting closer, but, again, that will be too short for Brighton," Frank said.
The Brentford boss also provided an update on record signing Igor Thiago, who has missed the last three matches in all competitions.
"Igor Thiago still has the joint infection. I don't know exactly a time frame... but it'll be weeks."
Frank is also without the services of Aaron Hickey, Rico Henry, Josh Dasilva and Gustavo Nunes, leaving the Brentford boss light on options ahead of a busy run of fixtures.
In positive news, Mads Roerslev is available for selection after Frank revealed his substitute appearance against Forest was cut slightly short as a precautionary measure.
Written by
Ben Sully