The 24-year-old had been at the club since the age of 11, and was a Fergie favourite as a youngster, but was allowed to leave to join Arsenal for a fee reported by Sky Sports to be £16 million before the summer transfer window shut.
Much was made of the fact United let one of England's young internationals join a direct rival, and van Gaal came out and bluntly told the press that Welbeck was allowed to leave because his goalscoring form wasn't up to scratch.
"After he came back from Sunderland, he does not have the record of Rooney or Van Persie, and that is the standard," said the Dutchman to Sky Sports at the time.
"And we have Falcao. Today, in training, he had one chance and it is a goal. That's why we had to let him (Welbeck) go.”
But given the seasons both Welbeck and Radamel Falcao have had, it is interesting to see that van Gaal has been proven right in his decision on one hand, and wrong in his judgement on another. Would the England international, being familiar to Old Trafford, have contributed more to United's season than the Colombian, who has netted just four times?
Currently Arsenal are ahead of United in the Premier League top four, with each side scoring wins over the other in their games this season. And Welbeck has played an important role in Arsenal's season in a variety of attacking roles.
Because of his work-ethic, energy and speed, Welbeck has constantly been utilised on the wing wherever he has played. Regardless of whether he wants to be the main man up front, the 24-year-old has found himself playing out wide at Arsenal recently, just like he did at United before.
For much of the Gunners' recent impressive league form, the England man has played left or right wing, and it is telling that he has created a goal a game in his last two starts, especially as each match was a 2-1 win.
However, Van Gaal's original comments that Welbeck didn't score enough hold some sway, as the forward is currently on eight goals for the season out of 33 games - a figure not dissimilar to the 10 in 36 he managed last year. Then the forward went through a purple patch in the middle of the season where he scored six in six games.
A key observation is that most of Welbeck's goals for Arsenal this season have come in earlier games where he functioned as a centre-forward, while his good form at United last year also came in that position. Crucially, those goals have generally come against mid-table opposition, and Welbeck has had further chances against tougher sides in that striker role for Arsenal where he has failed to find the back of the net.
He has proven that he offers some goalscoring threat against plenty of Premier League teams, just perhaps not the very best. Ironically, the one time Welbeck has scored against a team in the top seven was during his return to Old Trafford as the Gunners beat United in the FA Cup quarter-final.
But with Olivier Giroud having proved himself as the club's main striker, and claiming a position Welbeck would like for himself, it makes sense for Arsene Wenger to utilise him in other positions. Welbeck playing out wide was never an option this season at United because van Gaal doesn't tend to use wide attacking players in that way.
So the former Netherlands boss may perhaps feel he has been proven right, in that Welbeck has failed to demonstrate at Arsenal that he is a 20-goal-a-season striker. But considering the performances (or non-performances) of Falcao, who van Gaal so publicly compared to Welbeck when justifying selling the Englishman, as well as the versatility offered by the £16 million man at Arsenal, it appears Wenger might be happier with the way the deal has worked out than the United boss.