Marcelino departed on Thursday (AEST) despite leading the club to successive top-four finishes in the past two campaigns.
He also won the Copa del Rey with the club last term - beating Barcelona in the final - but following a reported falling out with owner Peter Lim he has been replaced by Albert Celades.
Garay, who started all three LaLiga games this season under Marcelino, made his feelings clear with an explosive social media post.
Después de muchos años contigo (no solo en Valencia) sé a la perfección cómo eres, tanto en lo profesional como en lo personal. Pero no solo yo me quedo con ello, se lo has demostrado a todo el mundo. Porque el camino se demuestra andando y tú has hecho un recorrido limpio, transparente y sano. Te vas por la puerta grande mister. Quien haya tomado esta decisión NO solo te llevó a ti por delante, arrastró a todo un equipo y afición, algo que alto y claro digo: NO ES JUSTO. #24
"After many years with you (not only in Valencia) I know perfect how you are, both professionally and personally," he wrote on Instagram.
"But not only I see it, you have shown it to everyone. Because the road is demonstrated by walking and you have made a clean, transparent and healthy route. You go through the big door mister.
"Whoever took this decision not only trampled over you, but over a whole squad and fanbase.
"I say it loud and clear: IT'S NOT FAIR."
Dani Parejo, Valencia's captain, also appeared unimpressed with the club's call to replace Marcelino.
"Mister, I wish you the best," Parejo wrote on Twitter alongside a photograph of him embracing the coach.
"I'm sure you'll be fine wherever you go. Thank you for making the club bigger and me a better player."