"You're talking months," he said. "Six weeks to heal and then a bit longer."

Fleck had been a concern for the Blades ahead of Friday's 1-1 draw at Luton, after which Heckingbottom revealed the full nature of the injury, sustained in last Saturday's 3-0 win over Blackburn.

"It's a massive blow. I love what he does and what he brings us," he added.

"He took some big knocks in the game and it wasn't settling down. So we sent him to be scanned and an X-ray showed up a fracture. He's going to be out for a while."

It is the second serious injury the Championship table-toppers have suffered this season following the loss of veteran striker Billy Sharp, who is out for an extended period with an ankle injury sustained against Middlesbrough.

Friday's point at Kenilworth Road left the Blades two points clear at the top - but in danger of being overtaken by Watford, Reading and Blackburn later on Saturday, if any of them win.