Nigel Pearson hailed Richard Wright's brilliant first half reaction save as a "defining moment" in Saints' final day win.
With Saints already trailing 1-0, Blades centre half Matthew Kilgallon fired in a bullet header at goal from a corner.
It looked destined for the back of the net and from 2-0 down at half-time Saints would have had a mountain to climb to avoid relegation.
But Wright, on loan from West Ham, instinctively stuck out a hand and somehow kept it out.
Saints went straight up the other end and saw Marek Saganowski head home at the far post to level at the break and put Pearson's men back on track.
Pearson said: "There were a number of defining moments for sure - plenty.
"Good players make good decisions and Richard was in the right place.
"It was a top save and a defining moment for us."
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