Salomon Rondon hit the only goal of the game to extend their unbeaten league run to four matches and make it 10 points from the last available 12 for Tony Pulis' side.
That streak has seen Albion pull away from relegation danger and up into mid-table with just nine matches remaining, and McAuley suggested that the impressive form is down to renewed energy levels amongst the players.
The Northern Ireland international also admitted that the club's glass ceiling has now been raised, and they have moved from looking to secure their top-flight status to targeting a club-record season.
McAuley told Sky Sports: "We have found our legs again and a bit of confidence with it and it's great to beat a team like Manchester United. As you can probably imagine, the boys are bouncing off the wall.
"We are really really happy with what we have done, we've got to 39 points with nine games to go. 40 points is the big one, that's the one that people think gets you safe. That was the big thing, so hopefully we can kick on for the rest of the season.
"Most of the lads are talking about bettering last year's points tally. With nine games left, it's possible to get to 50 points which the club hasn't done before."
Before their last four matches, from which they have three wins and a draw, the Baggies had slipped deep into trouble due to a run of just two victories in 12 league games.
However, they are now a huge 15 points clear of the drop zone, and Pulis' side can now relax into their season-ending run-in rather than endure the pressure of having to scrap for every point.