Ah yes, that's probably the best way to reintroducing it!
Gou hears rumors of an nearly unbeatable Greninja roaming around the the outskirts of Lumiose City and immediately sets out to investigate, dragging Ash along for the ride (and not listening to Ash's ramblings about "is that him?"). Upon their arrival to Lumiose City, Gou hears it's close by and he rushes out ahead of Ash, wanting to catch it first before he gets a chance and encounters on the outskirts of the city! All he needs is one good throw! Aaaaand, GO!
As the ball flies towards Greninja, it slices it in half with a well-timed Cut. Frustrated, Gou throws more and more balls, each of them cut down like the last, until he's down to his last one. Gou only has one more throw, and as if teasing him, Greninja allows it to make contact...only for Gou to find out it had a trainer all along! (Turns out none of the previous trainers tried chucking a ball at it before...)
Shook beyond belief, Gou falls to the ground on his knees as Greninja prepares to leave...until it senses a familiar presence. Curious as to why Greninja hasn't left yet, Gou looks up as he notices Ash and Pikachu finally catching up to him.
Gou sees Ash and the Greninja lock eyes with each other. Gou was about to warn him that the Greninja had a trainer already until he was stopped by Pikachu's unexpected squeal as it rushes towards the Greninja as if it was an old friend.
Ash and Greninja bring their right fists together in sync as Greninja suddenly became enveloped in a veil of water right before his eyes! Gou had never seen such a transformation before! When the water receded, he was met with the sight of a large shuriken upon the Greninja's back, who now shares a striking resemblance to the trainer in front of it.
Ash and Greninja slowly turn to Gou at the exact same time. Ash says, "Gou! Meet Greninja!"
has stopped working.
(sorry for the big wall. I kinda needed to get in a writing mood today)