But Ruddy, who had a loan spell at Motherwell, told the Scottish Sun: "You will always be a step behind if you're in the SPL trying to break into the England squad.
"That's no disrespect to the SPL because it helped me so much as a player and it's helped so many others. They say it's a goldfish bowl with Rangers and Celtic, but in the English Premier League everything's scrutinised.
"Highlights are shown in every country in the world, all the attention is magnified ten-fold. I definitely think Fraser has the potential to be a Premier League keeper.
"When I was younger people used to tell me I had the frame to really go a long way. Fraser's certainly got the frame and he's had two very good years at Celtic. He's capped it off with the title win.
"Now he will take that confidence into wherever he is next year and continue with that development."