City's Carling Cup campaign collapsed at the first hurdle as West Brom bounced back from a great Jo strike to reach the fourth round at the expense of last season's semi-finalists.
The rejuvenated Brazilian looked to have put the reshuffled Blues on the path to victory at The Hawthorns but goals from Gianni Zuiverloon and Simon Cox in three second-half minutes wrecked the Blues' ambitions.
Manager Roberto Mancini changed the whole team from Sunday's victory at Wigan, resting his preferred Premier League big guns such as Gareth Barry, Nigel de Jong, James Milner and skipper Carlos Tevez before facing Chelsea.
The Blues boss instead followed his policy of giving youth its chance with no fewer than four full City debuts for full-backs Javan Vidal and Greg Cunningham, centre-half Ben Mee and forward John Guidetti.
Dedryck Boyata, who enjoyed a successful spell in the side when Mancini arrived last winter, was also drafted into defence while Abdi Ibraham - who made his debut in the FA Cup last season - lined up in midfield.
There was also a belated first appearance of the season for Republic of Ireland goalkeeper Shay Given, who has lost his place in the Premier League side to new England keeper Joe Hart.
Albion, on something of a high after beating local rivals Birmingham 3-1 at the weekend, "only" made ten changes, with Nicky Shorey the last man standing from the side that Roberto di Matteo had fielded in the derby.
City might have taken the lead as early as the eighth minute when Roque Santa Cruz fed Jo down the right and the Brazilian whipped across a nice low centre that would have been easy meat had there been any takers.
But the Blues did go ahead after 18 minutes, and with a peach of a goal. Patrick Vieira delivered a superb ball up to John Guidetti and the Sweden Under-21 player flicked on for Jo to lash his shot first-time past Boaz Myhill.
It elicited the expected response from Albion, who launched a series of raids in retaliation only for Shay Given to deny Roman Bednar with a brilliant reflex one-handed save that had the travelling army singing his name.
Indeed, West Brom were unfortunate to find a goalkeeper in such determined mood as Given pulled off another marvellous save, diving to the foot of his right-hand post to scoop aside a diving header from Giles Barnes.
And when Cox managed to slip through an under-pressure back four - Vidal had already seen a yellow card - his composure failed him and he fired high into the crowd when faced with Given to beat.
But West Brom's start to the second half changed the complexion of the game and even Given couldn't resist as Zuiverloon thumped in a 55th-minute equaliser with Ibrahim bitterly complaining that he'd been fouled.
Worse was to follow for the Blues two minutes later as Cox beat the offside trap to engineer a one-on-one with the busy keeper and this time he held his nerve to beat Given and put the home side ahead.
With the tie slipping away, Mancini sent on Milner for Ibrahim and then with ten minutes replaced Santa Cruz with David Silva in a bid to force extra time, but Myhill defied Milner as time added on ticked away.