A brace from Mario Balotelli ended the Baggies' 11-month run without a home league defeat and was the second reversal in a week. The hosts - who also ended the match with 10-men after Youssuf Mulumbu was dismissed - were a distant second best and had no answer to the cast of richly assembled stars in front of them, with David Silva untouchable in the playmaker role.
West Brom boss Roberto Di Matteo's selection showcased the opposition's perceived current vulnerability as he paired Marc-Antoine Fortune and Simon Cox up front. September's Premier League player of the month, Peter Odemwingie, returned as a substitute after more than a month out with a knee injury.
Suspension to Gonzalo Jara forced the home side to field Nicky Shorey at his unaccustomed right-back position. Slovakia international Marek Cech came in from the cold to fill in on the opposite flank.
For City, Mancini made 10 changes from the team that lost to Lech Poznan in the Europa League on Thursday. Carlos Tevez went straight back into the starting selection at the expense of Emmanuel Adebayor after a dead leg and trip home to Argentina made him absent since he came off in the 53rd minute in the loss to Arsenal.
James Milner dropped to the bench on his return to the Midlands. The England international was joined by Adam Johnson and Aleksandar Kolarov, who remained shy of full match fitness after he damaged ankle ligaments on the opening day of the season.
The Baggies started the brightest on a crisp afternoon and Kolo Toure had to alert to stab the ball from Chris Brunt's feet when played through in the 3rd minute.
City's class then began to show as their big money buys got on the ball. After a sharp passing move, one of the most expensive, Mario Balotelli had the same stability as a cheap plastic chair when he fell down theatrically in the penalty area in the 11th minute.
From the resulting throw-in, a lay-off from Tevez found compatriot Pablo Zabaleta whose thunderbolt shot from 30 yards flew just wide of Scott Carson's goal.
City defied all the talk of dressing room unrest and continued to crank up the pressure. The visitor's skill and craft was on show in the 20th minute when Balotelli slid them in front.
The Italian was on the back post as Silva's beautifully disguised pass sent Tevez into space. His low cross found the controversial hit man.
Four minutes later, Carson had to be at his best as the influential Yaya Toure found Silva in space. The Spaniard fired a shot that had to be pushed away at full stretch.
Holes were now found with impunity in the Baggies rearguard and a second goal came in the 33rd minute. A chipped through ball from the unplayable Silva in the center circle was flicked into the air by Balotelli, who then spun and fired a rasping drive into the bottom right corner of Carson's net.
The Italian prospered from a weak header from center-back Gabriel Tamas after he had met the long ball with his back-heel.
The match continued in the same ilk until the half-time whistle blew as the Baggies failed to get to grips with their star studded opponents.
City continued the second half in control. Silva again sprung the Baggies back-line but Jerome Boateng couldn't quite beat Carson to his pass.
Out of the blue, however, in the 54th minute, Cox struck the outside of Joe Hart's post with a thumped effort from just outside the area. The England number one had no chance if it had been on target.
The game then sprung to life again in the 63rd minute when Balotelli was given his marching orders by referee Lee Probert. The Italian was handed a second yellow card after Mulumbu made the most of a gentle swipe.
West Brom had a route back into the match and Hart rose to tip a Nicky Shorey free-kick over the bar in the 66th minute.
The game was now an end-to-end contest. A goalmouth scramble saw a Tevez shot deflected to safety after Yaya Toure had charged into space in the 72nd minute.
After both sides then exchanged chances, Mulumbu followed Balotelli down the tunnel after he was handed a second caution for a lunge on Tevez in the 82nd minute.
As the match wound down to the end, opportunities were sparse with both sides a man light.
The final whistle handed the Blues a moral boosting victory to send them into the Manchester derby on Wednesday night.
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