Sheffield United's on-loan teenager Nathaniel Mendez-Laing has suffered a setback in his comeback from a hamstring injury.
The 19-year-old Wolves winger, signed at the start of the season on a five-month deal, was scheduled to return for the reserves against Nottingham Forest on Monday night, but was a late withdrawal.
Reserve team coach Chris Morgan told the club's official website: "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."
Meanwhile, defender Johnny Ertl took a big step closer to his return to first-team action by playing the full 90 minutes against Forest.
The versatile Austria international, 29, out since March with cruciate knee ligament damage, was close to returning to the bench for the npower League One win over Carlisle at the weekend.