They are even more vital now.
Simply put, Garry Monk's team showed exactly why United can't let the September 1 deadline pass without further additions.
Ander Herrera and Luke Shaw have already arrived at a cost of nearly £60 million.
But that Van Gaal had to give two youngsters their debuts in the opening game of the season should be a warning to the Dutchman that his squad is dangerously thin.
Tyler Blackett acquitted himself well. As did Jesse Lingard, before he was forced off with an injury during the first half.
But not even they will have expected to be so close to the first team this early in Van Gaal's reign. They got their chance because there is no one else.
Robin van Persie, Jonny Evans, Antonio Valencia, Michael Carrick, Rafael and Shaw were among those unavailable.
It's only taken a few niggling injuries to expose United's bare bones. The issue now is how to fix it.
They need a centre-back and another midfielder, at the very least. Without them, Van Gaal will face a battle to better last season's seventh-place finish.
But with one or two shrewd purchases, United can still look forward to the season with optimism.
They now have a manager who showed during the World Cup that he is one of the best around. But not even he can spin straw into gold.
It was perhaps a timely reminder that Van Gaal's appointment won't solve everything, and that David Moyes wasn't the only one at fault for United's miserable campaign last season.
The defeat to Swansea was a reality check. They are never nice, but it's better to get one after the first game than the 21st.
The truth is that United's destiny this season is still in their own hands. The squad needs improving, that much is clear.
The Premier League is decided by fine margins, and it would only take a couple of new players to make the squad, and United's chances this season, look a lot healthier.
But there's no guarantee.
The last weeks of the transfer window can be fraught and hectic. United know that better than most after their experiences last summer.
But after the defeat to Swansea, the pressure is on to put things right. And the clock is ticking.