The Blues full-back had a scan earlier this week, and medical staff feel that he may recover from the injury with just rest alone.
"Mitchell is going to have conservative treatment, which is basically rest and being in a surgical boot," said Clark.
"Four weeks yesterday, he’ll go back to see the specialist and hopefully that will have done the trick because that will take him up to six weeks since he did it. If it hasn’t he needs to have an operation and we won’t see him until pre-season. Even if he gets the all clear we’ll be pushing it to get him a game, because the timing is just return to training.”
Meanwhile Callum Reilly has recovered from the knee problem that saw him pull out of the Republic of Ireland squad two weeks ago and fellow midfielder Brian Howard is making progress with his own ankle problem.
"Callum Reilly (knee) joined the group today which is good news for us," said Clark. "Whether he’s done enough training to be in the 18 at Sheffield we’ll have to see. We're hopeful that Brian will return to training at the end of next week."
Blues are still without the services of Matt Green (knee), Akwasi Asante (hamstring) and Neal Eardley (knee).