Mendez-Laing, 19 was expected to be included in the squad to face Nottingham Forest Reserves yesterday (which ended 2-2) but pulled out at the eleventh hour through injury.
Reserves player/coach Chris Morgan told sufc.co.uk; "Nathaniel completed all his warm-up, but then just as we are about to kick-off he was stretching his hamstring and his knee locked.
"We got him back to Bramall Lane as quickly and as comfortably as possible for the doctor to look at him and until then there is nothing more we can say."