The goalkeeper signed a permanent deal with the Scottish champions in the summer after two seasons on loan from Newcastle United at Parkhead.
The 24-year-old told the Daily Mail: "It's hard to compare and contrast between Celtic and the Premier League. But Celtic has been fantastic for me and I can't thank the club and the coaching staff enough.
"The Premier League is a brilliant stage as the amount of money involved in it benefits everyone. But I needed to be playing.
"It's no good sitting on the bench, whatever league you are playing in, and I've no intention of being a No. 2 anywhere."
Forster also had loan spells at Norwich, Bristol Rovers and Stockport County while at Newcastle.