"Down" is relative to "up." 2018 is down relative to 2017. So yes, in the way that I meant it, 2018 was a down year - his performance went down from the year prior, just as was the case in 2016, 2013, and 2008. In each of those "down years," he went back up to a higher level in the next year.
This fits the context of what I'm saying. If 2019 is more of the same of 2018, I don't think retirement will be far behind. But if he goes back "up," then he might continue on longer. My point being, in each of he down-turns, or valleys, of his career, he was able to resurge and go back up. If he isn't able to do so in 2019, we'll be in uncharted territory and I would think the likelihood of retirement would greatly increase.