Rooney headed home the second goal in a 3-0 win over the struggling Baggies at the Hawthorns to help push the Red Devils towards a Europa League place.
Defender Phil Jones opened the scoring in the first half after he was found by Robin van Persie's cross and Danny Welbeck came off the bench to round off the afternoon.
England international Rooney - who was recently rewarded with a new £300,000-per-week deal - believes that the defending Premier League champions can use the win as a platform to build momentum.
Rooney said: "It is a confidence-booster.
"We've got Liverpool and Olympiakos next, where we're two goals behind [after the first leg], followed by West Ham and Manchester City. They're all big games.
"It's a good challenge and it's down to us as players to take that challenge on and show everyone we're a good team that means business."