David Moyes insisted Manchester United should be judged on how they "move on" after their worst start for 24 years.
Goals from Morgan Amalfitano and Saido Berahino for West Brom either side of a Wayne Rooney free-kick kept United on seven points, the same tally they had after six games of the 1989-90 season.
They now travel to Shakhtar Donetsk on Wednesday in the Champions League.
"You're always going to have bad results in football, it is how you deal with them," the Old Trafford boss said.
"We will move on and look forward to the next one. There are lots of games here and you get ready for the next one.
"I'm concerned after the game but only because we didn't play well. We can put it right."
Moyes, whose side have won just two Premier League games since he replaced the retired Sir Alex Ferguson in the summer, bemoaned the champions' below-par performance.
"We did not defend well at all but we did not attack well," the former Everton manager told BBC Sport. "We had a lot of the ball in the first half but didn't make the most of it."
On-loan Marseille midfielder Amalfitano opened the scoring in the second half when he ran who through the United defence before lifting the ball stylishly over United goalkeeper David De Gea.
Rooney responded for United when his free-kick - which was intended as a cross - found the net, but Berahino got the winner with a low strike as the hosts pushed for a second of their own.
"When we got back in I expected us to win it with the talent we have got," added Moyes.
"Our intensity was not good enough in the first half, we tried to pick it up in the second half but we got done by it as well [on the break]."
The win for the Baggies was just their second this season in the Premier League and their first on the road. It was also their first victory at Old Trafford since 1978.
Manager Steve Clarke was keen to praise his players rather than focus on United's poor display.
"We deserve a lot of credit," he said. "We came here to be positive. We made some good late additions in the transfer window and now hopefully the supporters and everyone else can see we can have another good season."