Martin was brought in as a free agent last summer to act as back-up for Jordan Archer. This means he has been limited to one league appearance this season.
The Lions have Harry Girling coming through the ranks, so he can act as back-up next season as well as playing in the cup competitions. Therefore, they won’t need Martin, so he could be released as a free agent again. It was always likely to be a short-term solution with little financial costs as Millwall navigated through their first season back in the Championship.