He didnt play in a 2 man midfield at Valencia. He played in a 3 man as the most advanced/attacking of the 3.
https://www.whoscored.com/Teams/55/Archive/Spain-Valencia?stageId=12647
I looked up his games for Barcelona and from the ones I looked at he was subbed on for Rakitic 2 times, started another and was subbed off for Mascherano, played right midfield and scored 2 goals with Busquets in CM, played right midfield with Busquets in CM again. So now im looking at his first games for Barcelona and he played left of the midfierld 3 with Busquets in CM, then right with Busquets in CM, then same again, and same again and then finally against City he was back on the left of the midfield 3 with Busquets in CM
So doesn't look like he was a replacement for Busquets at all, would have been a really odd choice in the first place but theres no evidence whatsoever from the lineups that he was playing his role or competing with him for it. Rakitic was the one he was being subbed with most but sometimes it was all of Gomes, Rakitic and Busquets playing.
Regardless here are some of his clips of where he did look like a player with potential for Valencia and in the areas he should be used.