BTW Isco under ZZ was not a key player.
I don't know what to tell you except that he absolutely was.
During the year that ZZ's Madrid won both the league and the CL (16/17), Isco was in probably the form of his life and Bale was, well, injured.
To begin the year, James was in front of Isco on the pecking order, but then Isco started pumping in MOTM performances for their "B" team (him, James, Kovacic, Vasquez, Danilo, Morata, etc.) and that "B" team won almost every single game in the league that they played - and won them comfortably.
It was the one time in the last few years when Real had the personnel to adapt and go from their usual giant-killing/big-game strategy with fast transition and quick off-ball movement, to short passes and 1-v-1 dribbling skills of Isco, Kovacic etc.... And they simply destroyed smaller teams in La Liga.
During that time, Isco scored some late winners, was absolutely phenomenal, and won himself a starter spot.
He started every big game from the 2nd leg vs Bayern, and was vital in the 2nd leg vs Atletico in the CL semi,
He would continue to hold to that starter spot through the 17-18 season as well (ending in Bale coming off the bench vs Liverpool).
So, after the 16/17 which was his real coming out party as a starter for Madrid, he stayed there as a starter, James left to Bayern and Bale was starting lesser games and coming off the bench in the big ones.
Isco started 10 games that year in the CL, including the knockout ones, and including the final again.
I honestly don't know where this ridiculous notion of Zidane hating Isco comes from, other than uninformed internet memes which might be referencing old stuff that was obviously either untrue or resolved.
Isco was asking for more playing time at the time, and he eventually earned it and got it.