Celtic need additions out wide after Roberts stated he would remain with Girona.
Allan McGregor and Scott Bain have been called up by Scotland after impressing for Rangers and Celtic.
Roberts has struggled in Spain since choosing to join Girona over Celtic.
Ajer underwent surgery on a broken eye socket suffered in Celtic's draw with Livingston.
Ex-Celtic playmaker Stuart Armstrong left the club this summer.
Celtic have had next to no luck with injury under Brendan Rodgers this season.
Armstrong left Celtic in the summer to join Southampton.
Christie was linked with moves away from Celtic, with Sunderland said to be interested.
Celtic were interested in McGinn last summer, but Villa won the race to land the midfielder.
Lustig's Celtic contract expires at the end of the season.
With Craig Gordon now aged 35, here are three potential replacements for the Scottish veteran at Celtic Park.
The Manchester United centre-back could be an ideal short-term option for Celtic.
Celtic have one of the most underrated wingers on their books and he is currently outperforming the Bhoys loanee.