This trade is rather straight up and one of those instances where it helps both teams accomplish what they need to accomplish. For the Dodgers, they would be acquiring a great right-handed hitting bat that absolutely demolishes left-handed pitching. For the Tigers, they would be getting a controllable young outfielder as well as a prospect to bank on.
This might seem like a small price to pay for someone that led the league in batting average against southpaws and it is; that is what makes Castellanos a great trade target. He will be a free agent after the 2019 season and has holes in his defense, which lowers his trade value.
A return of Pederson and Gray should be more than enough to make the Tigers budge. The team would essentially be trading one year of Castellanos for multiple years of Pederson — who had a WAR only 0.3 less than Castellanos, per FanGraphs.
Throw in Josiah Gray, who the team acquired from the Cincinnati Reds and the Tigers would be getting a more than suitable replacement as well as a nice prospect.
While the defense could be a concern for the Dodgers, as we saw with Matt Kemp, the team is great with positioning fielders in a certain way to mask any defensive inefficiencies.