Defender Virgil van Dijk and striker Teemu Pukki scored two apiece to ensure the Scottish champions will be facing either St Patrick's Athletic or Legia Warsaw in the next round after sealing a 5-0 aggregate triumph.
Media reports in the build-up to the second leg of the Champions League qualifier focused on a reported offer from Southampton for Forster.
But Deila revealed he had not felt the need to speak to the England international over his future.
He said: "We want to keep Fraser. Of course we want to qualify for the Champions League so we want to keep him for the Champions League, and even longer.
"He hasn't (indicated a desire to leave). He has done a good job since he has come to Celtic, but Celtic have also been fantastic for him and I hope that will continue because we need our best players and he is a fantastic keeper.
"Something will happen in the end but if it happens now or in the winter or next summer, I don't know, we will see. But we have to keep on going.
"Things are happening around my players all the time but Fraser is an unbelievable professional and I know that he really wants to play in the Champions League."
Deila was happy with his side's attitude, and laughed when it was suggested Celtic fans were unhappy when Pukki was replaced by Liam Henderson immediately after scoring his second goal.
"We wanted to see Kris Commons in the number nine role, but I will keep that in my mind for the next time, if they start booing,” said Deila. "Teemu was good. He had one or two chances before he scored.
"We started very well. The first 30 minutes was about killing the game and that's what we wanted to do with good tempo on the ball.
"We worked a lot on set play which will be important when we go further so two goals today was a bonus. We wanted to do the same things in the second half but I didn't think we managed that so that is what we have to work on.
"We have 90 minutes to try to really destroy the opponent, when you get three goals, you want to go for more."