The Blades have been rocked by injuries to several key players in recent weeks and Blackwell is hoping midfielder Montgomery has not damaged ligaments.
Blackwell said: "Monty's done his knee. We're not sure about the severity of it, but it doesn't look good.
"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."
Striker Jamie Ward, who strained his hamstring against derby rivals Sheffield Wednesday, has undergone surgery on a broken wrist, James Harper damaged his knee in last week's draw with Ipswich, defender Matthew Kilgallon is nursing an ankle problem and summer signing Lee Williamson has yet to feature this season after undergoing back surgery.