The Bhoys qualified for the Champions League group stages after a 3-2 aggregate win over Kazakhstani side Shakhter Karagandy, but have been drawn with Barcelona, AC Milan and Ajax in what is considered the 'Group of Death'.
But Forster is refusing to commit to the Scottish champions, citing his England international prospects as one of the main reasons.
He feels that prolonged involvement in the Champions League is the best way to play for England, and with Benfica getting drawn in a group with Paris St Germain, Olympiakos and Anderlecht, they appear to stand a better chance of progression.
Speaking at Wednesday's post-match press conference, the 25-year-old said: "You keep doing as much as you can and playing in the Champions League can only benefit me going towards that.
"I am happy with my England call-up and it gives me something to look forward to next week."
Despite the interest from Benfica, along with various English clubs, Forster has said he would find it 'very difficult' to leave: "I have said that from the moment I walked in the door.
"I have seen nothing like Champions League nights here.
"The fans tonight have shown that. It is a fantastic club to play for and the fans were the 12th man (against Shakhter) and that was what we needed.
"They helped us massively during the course of last season too."