The 27-year old hurt his knee in the 1-1 draw with Doncaster Rovers on the weekend and Blackwell is hoping for the best possible result.
"Monty's done his knee. We're not sure about the severity of it, but it doesn't look good," the Blades boss said.
"His knee is in a brace. He running one way and his knee got knocked the other side and he said his knee felt all wobbly and loose just before half-time.
"So it's not a good sign. Please God it's not what we think it is."