Referee Rob Styles had rival managers Neil Warnock and Rafael Benitez questioing his judgement as the new Premier League season started in controversy.
After a 12-year absence from the top flight United were denied what would have been a famous victory on their return by Hampshire official Styles.
Rob Hulse, a £2.2million summer buy from Leeds, had sent Blades fans into a frenzy by heading the opening goal of the campaign 52 seconds into the second half.
The game's flashpoint arrived in the 69th minute, with Robbie Fowler and Liverpool captain Steven Gerrard exchanging a one-two on the edge of the area.
There appeared to be no contact as Blades captain Chris Morgan slid in to make a challenge, with Gerrard's momentum as he clipped in a shot that was scooped up by Paddy Kenny sending him tumbling.
But Styles immediately pointed to the spot, allowing Fowler to send Kenny the wrong way.
Liverpool manager Benitez believes Morgan, who had been booked in the first half for a harsh handball decision, should have been sent off.
"It was a clear penalty, and a red card. It's as simple as that," insisted Benitez.
"The player said he felt contact, the ref said penalty, so it should be a straight red card because he is the last player."
Warnock stood up for Morgan, claiming: "I didn't think it was a penalty. There was no contact. Apparently the ref has said there was intent, so that means there are going to be a lot of free-kicks given!
