Charlie Austin gave Southampton victory in the 87th minute, meeting James Ward-Prowse’s right-wing free-kick with an unerring header in his first appearance since signing from Queens Park Rangers.
A beaming Austin said: “It does not get any better than that.”
In a further blow to Van Gaal, right-back Matteo Darmian was forced off after a second-half collision with Shane Long, exacerbating United’s defensive injury crisis.
United are now 10 points below Leicester, five points off the Champions League places and only three points above Liverpool, who saw off Norwich in a dizzying game thanks to a 95th-minute winner from Adam Lallana.
After Roberto Firmino had put Liverpool ahead from James Milner’s pass in the 18th minute, Norwich hit back with a clever Dieumerci Mbokani back-heel, a goal by debutant Steven Naismith and a Wes Hoolahan penalty.
But goals from captain Jordan Henderson and Firmino drew Liverpool level and although Milner’s 75th-minute effort was cancelled out by Sebastien Bassong’s drive in the 91st minute, Lallana had the final word.
“At the end we can be really happy with nearly everything, especially with the three points,” said Liverpool manager Jurgen Klopp, who lost his glasses in the touchline celebrations after Lallana’s winner.
“I usually have a second pair of glasses, but I can’t find them because it’s hard to find glasses without glasses!” he joked.
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