The Leicester game was aroud the time his performances tailed off. You can choose to use that as a measuring stick, or you could look at the games vs. Madrid when he was incredible, or even the fact Sevilla were legitimately challenging for the title until he dropped off.
At times last season he was absolutely phenomenal, in the best form of his life, Sampaoli giving him this ridiculous free role where he would play wherever they needed him, starting moves off at the back, popping up in the box, creating overloads on the wing, he was running the games against Real where Sevilla were the better side.
The question really is why did he fall off so drastically, and is it because after a year out injured he just didn't have the legs to keep up Sampaoli's insane pressing and high tempo for the whole season, which I think is the case.
Look back to before the Leicester game last season and you'll see loads of articles like this one -
https://www.theguardian.com/football/2017/feb/19/samir-nasri-sevilla-leicester-city-champions-league
Where Nasri is described as, aside from Messi, the stand out player of the first half of the Spanish season, or Sampaoli describing him as the guy who gives the team oxygen.
There's no doubt his ability is still there.