If we hadn't wasted our huge lead, then we would have had more than enough opportunities to not play our best 11. Besides, Arda couldn't play the first half of the season, which is exactly when most of the mid-week Copa games and opportunities to rotate happen. And I'd have no problem giving Arda minutes ahead of Sergi Roberto, now that they're both here.
You're being unjust. Arda hasn't "been playing for five months". He's only been available to be played in the last five months. If one of our starter midfielders was unavailable, Arda would play much more frequently, and he'd adapt more quickly, simply because of the minutes spent on pitch. It's simple math. You can't adapt if you don't play. I'm not saying that he SHOULD play instead of someone, I'm just saying that he can't improve if he doesn't.