The main reason Kelly was one of the players sent to Arizona was the fact that Yadier Molina was taking away consistent playing time from him, as he only played in 63 games across 3 seasons (2016-2018), and only hit .154 with zero home runs in those games. He was the starting catcher for Arizona in 2019 and he hit 18 home runs with 47 RBI in 111 games. He was never able to put up huge offensive numbers in Arizona partially due to injuries, including a forearm injury that took most of his season this year, it was the 5th time in the last 3 years that Kelly landed on the IL.
And with the strong play of rookie Gabriel Moreno behind the plate, when Kelly came back from the IL he was not contributing at the plate and was losing playing time. So the Diamondbacks DFA'd Kelly in the middle of August. Just a few days later Kelly signed with the Tigers and finished the season with them, hitting just .173 in 18 games. Even though Kelly was released by Arizona a couple of months back, since he was on the 40-man roster and played 32 games with the D'Backs this season, he can still receive a World Series ring this season.