His second-half strike completed a comeback that had been started by Argentinian ace Sergio Aguero, who celebrated his 200th City appearance with his 130th goal, this one from the penalty spot.
West Brom made City work incredibly hard for the victory, with Stephane Sessegnon’s early thunderbolt giving Tony Pulis’ tenacious side something to hold onto.
City, sparked in part by second-half substitutes Yaya Toure and Kevin De Bruyne, eventually loosened the Baggies’ grip and prised the points away.
Nasri fully deserves his share of the glory; he was impressive throughout after stepping into the creative role vacated by the injured David Silva, while Aguero’s brilliance repeatedly lit up the greyest of Mancunian afternoons.
The win draws City to within two points of third-placed Arsenal, four points clear of United – they play at Tottenham on Sunday – and now five ahead of sixth-placed West Ham.
It also keeps up the momentum ahead of Tuesday’s quarter-final second-leg against PSG.
Manuel Pellegrini made five changes to the side that drew in Wednesday’s Paris thriller, perhaps with one eye on Tuesday’s return at the Etihad Stadium.
Nasri made his first start since September after recovering from a thigh injury, while Fabian Delph was in the first XI after shaking off his Achilles problem.
Elsewhere, Wilfried Bony played up front as the formation switched to 4-4-2, while there was rotation at full back with Aleksandar Kolarov and Pablo Zabaleta coming back in.
An intense pre-match downpour made for a slick surface and a fast start, with Aleksandar Kolarov and Nasri linking well down the left early on.
Just six minutes were on the clock when the home faithful were stunned into silence. Sessegnon collected a cross from James McClean and lashed it through Kolarov’s legs and past Joe Hart on the half-volley.
Sessegnon was causing trouble shortly after, fizzing a free-kick across City’s six-yard box, but nobody found a decisive touch.
City settled after the early setback, with Kolarov back on the offensive and firing in a series of crosses into the Baggies box.
It was the Serbian who won City’s penalty, going down under the challenge of Sessegnon after referee Mike Jones had earlier played advantage to City after a challenge on Aguero.
The Argentinian picked himself up off the floor to slot in a precision spot-kick past Ben Foster to mark his milestone appearance.
West Brom suffered another setback when striker Salomon Rondon was forced off injured to be replaced by Craig Gardner.
The substitute made a definite impact though, adding snap to the visitors’ midfield, blasting one free-kick against the City wall, and dragging another left-footed effort wide with Hart scrambling.
Despite West Brom’s endeavour – and there was plenty of it as you would expect from a Pulis side – City were winning the arm wrestle.
Nasri was seeing plenty of ball, the Frenchman dropping into midfield pockets along with Aguero to prompt his team-mates into action.
Wilfried Bony headed a half-chance wide in what was to be the first half’s final chance.
The second-half started with City firmly on the front foot, but more often than not facing a wall of 10 Baggies players to break down.
Aguero worked Foster with a skidding shot from 25 yards as City tried to up the tempo, while at the other end Sandro went down claiming for a penalty under Mangala’s challenge, with nothing given.
With an hour on the clock Pellegrini made his move, bringing on De Bruyne and Toure for Bony and Delph.
The duo made an instant impact, Toure directing play from the base of midfield before De Bruyne played a key role in the second goal.
He broke down the right, fed Navas, who nudged the ball onto Aguero. After he had two attemtps foiled Nasri showed great composure to control and then finish from the rebound, sending a roar ringing around the Etihad.
City stormed forward in search of a third, with Navas’ inviting centre rolling across the six-yard line untouched.
Aguero then had a great chance to make the game safe but he was brilliantly denied by Foster one-on-one, and the points were by no means safe heading into the last 10 with West Brom looking dangerous going forward.
Pulis put 16-year-old Jonathan Leko on to try and salvage something, and Gardner stung the tips of Hart’s finger-tips with a bullet free-kick from 30 yards.
Aguero came off to a huge ovation late on as City held on for the win, despite McClean’s late volley whistling inches past the post.
