Emnes raced clear in the 89th minute to net Boro's winner just when it looked like the points would be shared after Matthew Lowton`s first-half header had earlier cancelled out Matthew Bates' 18th-minute strike.
The result leaves the Blades bottom of the Championship table, seven points from safety, whereas Middlesbrough are now 11 points clear of the relegation zone after they extended their unbeaten run to five matches.
Adams said: "It's a cruel game - I feel as though we've been mugged, really. There are a lot of disappointed lads in the dressing room.
"We had chances throughout the game and caused them all sorts of problems. We feel particularly hard done by regarding a couple of incidents involving the referee. There's no consistency there. I think he'll look back at it and think he should have done better. The lad (Marvin Emnes) who scored the winner shouldn't have even been on the pitch because he kicked out at Michael Doyle."
On his side's chance of avoiding relegation, Adams added: "I think I'd look daft if I said we hadn't got a problem. We've got to believe that we can get out of it, starting with Tuesday's game against Cardiff, although it does look an impossible task."
Source: PA
Source: PA