Wayne Rooney scored from a free kick for the third straight league game in the 57th to cancel out Morgan Amalfitano's stylish solo goal for the visitors three minutes earlier, but United was run ragged at times. Moyes' decision to rest key players ahead of the Champions League next week badly backfired.
United has just seven points from a possible 18 and has already lost three league games including the 4-1 trouncing by Manchester City last Sunday to pile the pressure on Moyes. First-place Arsenal is already eight points clear of United, which is in 12th place in the 20-team standings.
Boos rang out at the final whistle, with United having made its worst six-game start in the league since 1989. West Brom hadn't previously won at Old Trafford in 35 years.
"I'm concerned after today's result, but only because we didn't play well," said Moyes, whose team looked to have turned a corner after beating Liverpool 1-0 in the League Cup on Wednesday. "But there are a lot of games to go and I'll try to put it right in the games to come."
In the northwest of England for a charity function, former world heavyweight champion Evander Holyfield was a special guest at Old Trafford and entertained the crowd in a pre-match interview on the pitch. But United lacked any punch throughout to further highlight the team's attacking deficiencies so far this season.
United hasn't scored in open play since the opening day of the campaign and looked lightweight going forward without the threat of Robin van Persie, who came on for the final half-hour to little effect after recovering from a groin injury.
West Brom played some brilliant football at times, its slick approach play often embarrassing a United defense that was missing the rested Nemanja Vidic, Patrice Evra and Rafael da Silva with Wednesday's Champions League match at Shakhtar Donetsk in mind.
"I thought we were tremendous from start to finish," West Brom head coach Steve Clarke said. "We did everything almost perfectly."
West Brom had the better first-half chances, with substitute Berahino heading a golden opportunity wide and Stephane Sessegnon blasting a volley over from close range, but they picked holes in United's defense with alarming regularity after the break.
Minutes after Sessegnon sashayed his way through some flimsy attempted tackles to set up a chance for Berahino, Amalfitano embarked on a similar solo run and this time, there was an end product.
After breezing past Michael Carrick and nutmegging Rio Ferdinand, the on-loan Marseille midfielder had the composure to wait for goalkeeper David de Gea to make the first move before chipping the ball in over the goalie.
The lead lasted barely three minutes as Rooney bent in a low free kick that evaded everyone, including a stooping Javier Hernandez, and crept into the far corner. United fans were further encouraged by the immediate introduction of Van Persie but they were soon out of their seats in disgust as Amalfitano applied a deft lay-off to Berahino, who fired a low left-footed shot into the corner from the edge of the area.
Substitute Marouane Fellaini had a goal disallowed for offside in the closing stages, one of the few decent chances United created in the whole match.
"You're always going to have bad results," Moyes said. "It is how you deal with them."
