The veteran former Chelsea goalkeeper has struggled to adapt to Unai Emery's new style of playing out from the back. The Czech Republic international nearly passed the ball into his own net against Manchester City, and also nearly gifted Cardiff's Harry Arter a goal at the weekend after easily giving the ball away.
Despite the few errors, Foster believes that Cech, who is statistically the best goalkeeper to have played in the Premier League, is being unfairly criticised.
"Cech is one of the best goalkeepers the Premier League has seen but that really is not his game," Foster said.
Petr Cech has the most number of clean sheets in Premier League history, amassing 201 over 440 appearances.
Speaking on 'BBC Radio 5 live', the former England goalkeeper Foster said: "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," added Foster.
Foster's Watford have made a strong start to the new season, winning all four of their opening games.