England keeper Butland hasn't played since fracturing his right ankle playing for England in the win over Germany in March.
He had surgery on Monday to clean up scar tissue following the initial surgery on the injury.
He broke down again in training on the eve of the season but assistant boss Mark Bowen says specialists are pleased with what they found.
Bowen said: “We can't put any timescale on it but the surgeon says it's gone well, which is good news. It will be a number of weeks, certainly.
“With all operations, the surgeon goes in and might find things that are worse, but they found what they expected and that's good."