There's a variety of factors. First of all, small sample size, he suddenly learned how to throw strikes but it's been said that he didn't really change much in his motion so he might revert back to his minor league walk rates, against much better hitters obviously which could be trouble. Despite his great stuff, his rate of inducing swinging strikes was very poor, and this was as a reliever, if he can barely get guys swinging-and-missing now then he will suffer as a starter as his stuff inevitably downplays in longer stints. Back to his stuff, he clearly has plus pitches, but the team completely gave up on him using his secondary pitches near the end of the season, whether this was a result of Navarro not being able to handle it, I don't know, but it's concerning if he's literally a fastball/sinker only guy.