Premier League - United overcome De Gea howler
The win came at a heavy cost for United, who lost Nemanja Vidic and Rio Ferdinand to injury, while De Gea's confidence looked rock-bottom after his blunder allowed Long's tame shot to trickle past him.
United started brilliantly, whisking the ball around in a flurry of movement. Former United goalkeeper Ben Foster looked hesitant when faced with a barrage of crosses, and it came as no surprise when Rooney put the visitors in front.
Ashley Young squared from the left to Rooney, who killed the ball, and drilled a low, left-foot shot into the right corner of Foster's goal - it was his 11th Premier League goal of 2011, a figure exceeded only by Arsenal's Robin van Persie.
This was United's new generation at their best - Welbeck and Tom Cleverley were preferred to Dimitar Berbatov and Michael Carrick, and the result was a youthful vigour with which West Brom could not cope.
The average age of United's starting XI was 23.9, the lowest in the Premier League on the opening weekend. It is even more remarkable considering the team that started the final game of last season against Blackpool was 28.5 on average.
The replacement of 40-year-old Edwin van der Sar with a man half his age accounts for much of that change, but De Gea showed the downside of inexperience, producing a howler for the second Sunday in succession.
At the Community Shield, he was beaten by a long shot from Manchester City's Edin Dzeko, but the error that gifted Long his goal was even worse.
The Irishman hit a weak, low shot from a tight angle on the left, which somehow squirmed under the falling De Gea's body.
Encouraged by this unexpected filip, the Baggies continued to press, and Somen Tchoyi almost got lucky with a mis-hit cross that De Gea clawed away from under his crossbar.
By now, the Spaniard's every touch was being greeted with a roar of expectancy from the home fans, and although Ferguson resisted the temptation to replace his goalkeeper, De Gea wore the look of a condemned man throughout the second half.
Roy Hodgson has infused West Brom with pragmatism, and they defended effectively in numbers, allowing moments of quality from Long and Tchoyi to cause problems at the other end.
Vidic and Ferdinand both limped off - the latter to a suspected hamstring strain - meaning the visitors ended the game with none of the back five that played much of last season. Patrice Evra and Rafael are also injured, while Van der Sar has retired.
Young was United's most consistent attacking threat, and it was fitting that he should conjure the winning goal. He darted inside from the left, reached the byline and played the ball across goal - it went in via two defenders, though the own-goal will be 'credited' to Reid who got the final, decisive touch.