Nasri needed around 100 stitches after suffering a freak training ground injury back in October, with the subsequent surgery ruling him out for almost six months.
The Frenchman's tendon had detached from his thigh bone, giving club doctors the impression that he would not be back in contention for the Champions League quarter-finals even during consultations throughout March.
But he has stepped up his training over the past few weeks, meaning Pellegrini's decision to cut him from his European squad - with Kelechi Iheanacho entering the list in his place - seems especially unfortunate given the Blues host Paris Saint-Germain on Tuesday hopeful of booking a place in the semis.
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The City boss revealed that Nasri will most likely start against West Brom on Saturday, with David Silva sitting out as he prepares to face PSG in top shape.
"Really it was a very unlucky injury because it was in a training session without any problem," the Chilean told the press. "He shot a ball and he felt that something happened in his muscle, and it was a very hard injury.
"I am happy for him because he worked very well in the last month, he returned maybe a little bit more early than the doctor expected, that's why he is not in the Champions League list, I changed him for Kelechi.
"I am very happy I am telling you about him and I hope in the last games of the Premier League he will demonstrate how good a player he is."
Vincent Kompany, meanwhile, is battling to be fit for the clash with the French champions after suffering a calf injury in the previous round against Dynamo Kyiv.
The Belgian is not in contention to face West Brom, but could yet recover in time for Tuesday's game.
"I cannot answer you three days before the game," Pellegrini added, "because the way he improves Saturday, Sunday, Monday, it's three days that for him are very important.
"It's 72 hours that he must have a lot of tests to see if he's 100 percent ready for the game."