The Red Devils succumbed to the Baggies 1-0 at The Hawthorns after Juan Mata was sent off for two yellow cards in the first half before Salomon Rondon scored the only goal of the contest in the 66th minute.
The defeat comes at a bad time for United, who had won four matches in a row in all competitions prior, as they take on arch enemies Liverpool in the Europa League Round of 16 first leg at Anfield on Thursday and van Gaal hopes the result does not have an ill-effect on his side who are also chasing a top four finish in the Premier League.
"I think always this match has an influence on the next match so it is, for us, very bad," van Gaal said. "It is a bad moment and it wasn't in my opinion a necessary loss.
"I don't want to say that we deserved to win, no, but we don't deserve to lose.
"When you see at the end the chances that West Bromwich Albion has created in comparison with us, it was an unnecessary loss.
"When it is a draw and you play 60 minutes with 10 men, you can accept it - but now you lose and you lose contact with the first four or five."