Ten days after his three-at-the-back experiment in City's shattering Champions League defeat at Ajax, manager Mancini was at it again at the other end of the pitch versus West Ham United at Upton Park.
His decision to start with Mario Balotelli, Carlos Tevez and Edin Dzeko in the same XI for the first time failed just as spectacularly as the gambit versus Ajax as the champions failed to score for the first time in almost seven months.
It could have been even worse for Mancini's men had Kevin Nolan not seen an early goal controversially ruled out for offside as West Ham more than held their own and were marshalled brilliantly by Winston Reid and James Tomkins.
Mancini yesterday mocked suggestions he could become a victim of the swelling ex-Barcelona influence at City - director of football Txiki Begiristain and chief executive Ferran Soriano - amid rumours Pep Guardiola may be lined up to replace him.
He refused to deny the reports he came close to agreeing to join Monaco before City's extraordinary title triumph.
He proved he was a risk taker at both ends of the pitch today, handing Kolo Toure his first league start since August and recalling James Milner, although he was forced into a last-minute change when the latter hurt his hamstring in the warm-up and was replaced by Gareth Barry.
That may have sunk Mancini's Plan A but there were no excuses for the shocking defending that should have seen them behind inside four minutes.
Mark Noble played a simple straight free-kick over the top, which Nolan brilliantly volleyed past Joe Hart.
The assistant's flag was up instantly, although replays showed Nolan may have been level.
Five minutes later and City should have been bemoaning an offside flag but Dzeko headed a simple rebound wide, anyway, after Jussi Jaaskelainen parried Samir Nasri's stinger.
Andy Carroll also went close to his customary goal against City, whose new-look attack continued to misfire.
The same could be said of the assistant referees, who spared Balotelli embarrassment when they controversially flagged again after the striker badly shinned Tevez's free-kick from point-blank range.
Jaaskelainen almost fumbled Tevez's snap shot after the restart before getting away with spilling the striker's corner as City threatened, while James Collins was also booked for a cynical check on Dzeko. The match saw one of the misses of the season midway through the half from Barry, who controlled Tevez's ball on his chest put somehow prodded wide from barely three yards.
Mancini had seen enough and hauled off Balotelli for Sergio Aguero, with the Hammers manager Sam Allardyce sending on Gary O'Neil for Diame.
Suddenly, City were swarming but Reid and Collins were producing heroics at the back.
West Ham were also a threat on the break, Carroll's lunging finish after more fine work from Jarvis his final act before the introduction of Carlton Cole.
O'Neil and George McCartney were both booked and Aguero almost caught out Jaaskelainen from a quick free-kick.
Tevez and Collins were replaced by Javi Garcia and Jordan Spence, while Mancini played his last card when he threw Scott Sinclair on for Nasri, to no avail.