"It will keep him out for a few weeks, it will be weeks rather than months."
On-loan Leicester defender Harry Worley, who came on for Mitchel-King on Saturday, is his likely replacement.
"We're going to need some of the players off the bench," said Gradi.
"Harry is very receptive, he's dying to learn and improve, and he's super to work with.
"Clayton [Donaldson] got a whack on the hip, so we won't be able to evaluate that for a couple of days. Joel [Grant] was just tired."
Grant scored the second goal of the afternoon, which was sandwiched in between two goals from top scorer Calvin Zola.
The Congo striker took his personal tally for the season to 16 on his return to the side, having missed two games with an eye injury.