Salomon Rondon scored the only goal of the game as West Brom defeated 10-man Manchester United 1-0, dealing a blow to the visitors' top-four hopes.
United came into the game buoyed by four wins in a row in all competitions, but their task at The Hawthorns was made far tougher when Juan Mata picked up the first Premier League red card of his career with two quick-fire bookings in the first half.
Louis van Gaal's side responded well to that setback and came close to opening the scoring through young Marcus Rashford as they looked to counter West Brom, who struggled to carve out many meaningful chances despite their advantage.
Yet Tony Pulis' men found their breakthrough midway through the second half thanks to Rondon's clever finish from close range - his fourth goal in six league matches.
Van Gaal's side pushed for an equaliser, but diligent West Brom defending ensured the visitors remain sixth in the table and three points behind Manchester City in fourth, having played a game more.
United, who welcomed back Chris Smalling, Matteo Darmian and Michael Carrick to the starting line-up, started brightly, but Craig Dawson missed the first decent chance when he headed wide following Rondon's teasing cross.
The home side began to look an increasing threat on the break and from set-pieces as United laboured in possession, with belief among the home fans growing further when Mata was sent off for two yellow cards picked up in the space of three minutes - the first for blocking a quick free-kick, the second for a late swing of his left foot to bring down former team-mate Darren Fletcher.
United were forced onto the defensive, but, barring a low drive from Saido Berahino which David de Gea saved comfortably, West Brom were struggling to make their numerical advantage count.
Indeed, it was United who next came closest to breaking the deadlock just after half-time, as Rashford's low shot from the edge of the area deflected narrowly wide of Ben Foster's left-hand post after a strong run from Anthony Martial, before Jesse Lingard clipped the top of the net with a looping drive from 25 yards.
Berahino stabbed substitute Sebastien Pocognoli's cross narrowly over the crossbar, but United failed to heed the warning as the full-back picked out Rondon in space just six yards out and the Venezuelan controlled the ball superbly before drilling low into the corner past De Gea in the 67th minute.
Van Gaal threw on Memphis Depay in hope of a similar comeback to that produced by 10-man Liverpool at Crystal Palace earlier in the day, but West Brom held on for a win that takes them to within a point of the top 10 and leaves United facing an uphill battle to keep pace with the top four.