But Gregory has managed to secure the signing of Finnish centre-back Sampsa Timoska, who will be available from next month.
Gregory told The Gazette: "We've got 35 players and that's an enormous amount.
"We need to reduce the size of the squad but a contract is a contract and has to be honoured.
"We've got the luxury of a lot of strikers, but in terms of defenders, we're not so blessed.
"The young lads Shimmin and Kanyuka are out and there's not a lot of room for manoeuvre. It is an area we do need to strengthen."
He added: "We've certainly got a lot of work to do to stay up."But myself and (assistant boss) Richard Hill have been in this type of situation before at Wycombe and certainly aren't giving up hope.
"Wycombe was tough, and if all was rosy here then I wouldn't be in the job now - and I'm enjoying the job.
"We just need to take a deep breath and get back to work at Stoke on Saturday."