The 36-year-old has made several mistakes so far this season as he attempts to adjust to playing out from the back, the option preferred by new Gunners manager Unai Emery.
But Foster sympathises with the Czech for having to change his game so drastically late on his career.
Speaking on BBC Radio 5, Foster said: "Cech is one of the best goalkeepers the Premier League has seen but that really is not his game.
"He has spent his whole career playing one style of football and now, at the end of his career, he's being asked to play another one altogether.
"His strong points have always been being big and commanding and making saves, but the way football is going now you almost have to be one of those Ederson-type of goalkeepers.
"It's going to take some time to get used to and I do feel sorry for him."