The England goalkeeper played in Tuesday's 4-0 victory over Icelandic outfit KR, which secured a 5-0 aggregate win and progression to the third qualifying round of the UEFA Champions League, just hours after Celtic had knocked back a reported £7.5 million offer for his services.
Scottish champions Celtic are understood to believe Forster is worth much more, but Deila knows his club will not be able to hold onto the highly-rated gloveman forever.
"In the end, something will happen with Fraser," Deila said.
"But whether it happens now, in the winter or next summer, I don't know.
"I want to keep him because I want to qualify for the Champions League and build this team further.
"We want to keep him here. He has given me no indication he wants to leave. He is an unbelievable professional and I know he wants to play in the Champions League."
The Newcastle United youth product was part of Roy Hodgson's 23-man squad for the FIFA World Cup in Brazil but has only played two matches for England.
