Forrest has previously stated that he wanted to stay at Celtic, but now it seems the Scotland international is ready to leave Parkhead.
Forrest has rejected an offer of a new four-year contract.
“We offered him a contract and he has turned it down. He has told us he wishes to see what his other options are,” Deila told the Evening Times.
“I cannot see that we will go back to him with another improved deal because to my mind it was a very good offer that the club put to him.”