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The Yorkshire-born central-midfielder had to be substituted at the interval in the weekend's game against Doncaster Rovers. Montgomery, 28 this month, showed a ray of light in the specialist's report when medial ligament damage was diagnosed rather than a cruciate ligament problem - this will result in two months on the sidelines which is bad news for both Monty and the Blades.
Sam Ellis, United's assistant manager, said: "It is still a long-term injury and Nick will be a big loss to us. It will probably be early December before we see him playing for us again."
James Harper, a central-midfielder himself of course, missed Saturday's clash against Rovers due to an injury. Lee Williamson is also a long-term absentee.