The defeat left United six points behind Manchester City in fourth, having played a game more, and Van Gaal let his players know they have squandered too many points recently in the race to qualify for next season's UEFA Champions League, per Simon Stone of BBC Sport:
The Dutchman is viewing winning this season's UEFA Europa League as United's best hope of playing in Europe's premier club competition next term. But Van Gaal still preached caution about assuming United will boss the second-tier tournament in an interview with BT Sport (h/t Goal's Robin Bairner): "I think after today the Europa League's the best route but it will not be so easy because there are a lot of very good teams in it."
Van Gaal wants his players to face facts and acknowledge the chance to secure a top-four finish might already be gone:
It seems captain Wayne Rooney is already in agreement with his manager's way of thinking:
Such skepticism is perhaps understandable given the chance United wasted at the Stadium of Light. With the top four playing each other in Sunday's fixtures, the Red Devils could have reduced the gap to three points.
As it is, United's best-case scenario is remaining six points adrift with only 12 matches left. That would require Tottenham Hotspur to win away at United's neighbours, Manchester City.
But Van Gaal and Rooney will know it's United's own performances that will determine the Old Trafford club's finish this season. The display wasn't good enough at Sunderland, where Van Gaal thought his players lost the physical battles:
It's strange United couldn't seize control of the game when Van Gaal had two holding players, Michael Carrick and Morgan Schneiderlin, in midfield.
But perhaps the biggest disappointment was the inconsistency of United's attacking talents. Despite scoring, Anthony Martial went missing after a bright start, while Jesse Lingard and Juan Mata didn't do enough. That left Rooney to try and lead the line by himself, which proved too tough a task for the 30-year-old.
As for Van Gaal, he'll know only a top-four finish will save his job at Old Trafford. His team has almost no margin for error in the final 12 games.