Rough hopes Celtic can sign Forster permanently this summer.
He told the Scottish Sun: "I think young Forster has done really well.
"I was only at Celtic for a year-and-a-half but it's an incredible thing to try and go in and play for the club.
"Forster came in and commanded his position all season.
"Pulling that jersey on is really tough because you're expected to win every single week.
"Mentally you have to be tough and I think he'll be strong enough to cope with the pressure.
"Forster could definitely be a Celtic No1."