It is still early. In 2006 there was a young 26-yr old Tiger with very similar stats around May 9th, and he was replaced by Sean Casey by the end of July.
Castellanos is currently ranked 26th in UZR/150. So metrically speaking, if he improved defensively it hasn't been by much.
Offensively, he has been above rational expectations. He is carrying a very high and unsustainable BABIP, especially against RHP.
He is hitting very well and that is good for the Tigers and they should ride him while he is hot, as they should any player who is hot. But making any judgements one way or the other this soon in a season is just folly.
He's been hitting pretty well since June of last year. Almost a full calendar year.