The 20-year-old moved to the Britannia Stadium from Birmingham in January but was immediately loaned back to the Blues for the remainder of the season.
However, Butland's agent has warned that the goalkeeper wants to be playing regular football and will not be happy on the bench at Stoke, warning there will be plenty of interest from abroad in the youngster.
Mellor told 6 Towns Radio: "If Asmir [Begovic] does go it means Stoke have got somebody for the future. It might not be this season, it could be the season after.
"When I was in Barcelona a couple of Spanish clubs wanted to talk to me about taking Jack.
"We have to weigh up all possibilities and I have to keep him playing so if Asmir is still here on September 2nd I'm hopeful that Jack isn't. I don't see him sitting on the bench at Stoke City.
"Mark Hughes might see that differently and that's when Stoke and I do have issues. I have to keep Jack playing so that his international ambitions are on track and he continues to develop. There's nothing like actual games to help you develop, especially as a young goalkeeper."
Mellor also revealed that Chelsea were not the only top club interested in signing Butland in January, and claimed that Steven Gerrard was a big fan of the goalkeeper.
"Obviously it was well documented that we turned down Chelsea," Mellor said. "We turned down other top-four teams; there probably wasn't any of the big six that didn't show an interest.
"I had a meeting with Liverpool about Jack, and [Liverpool's managing director] Ian Ayre said the only player Steven Gerrard has ever picked up the phone and called them to sign was Jack when he was in the England squad in Poland and Ukraine last summer.
"That shows how highly thought of he is, and I think Stoke have got an absolute steal."