Per Fancred's Jon Heyman, the deal is worth $5.25 million with $250,000 in incentives.
Mercer, 32, had a decent 2017 season, hitting .255 with 14 home runs and 58 RBI. He played in 145 games for the Pittsburgh Pirates while serving as a solid veteran stopgap for the team at shortstop while it waited on youngsters like Kevin Newman or Cole Tucker to hit the big leagues.
He had a more modest 2018, hitting .251 with six homers and 39 RBI in 117 appearances.
Mercer is a decent fielder as well, though he isn't a game-changer in that regard. Nonetheless, he's an affordable option for the Tigers as they balance the need to remain competitive in the short term with the desire to keep their farm system stocked and their payroll manageable.
Indeed, he'll give the team a reliable, if unspectacular, option at shortstop in 2019 until the younger options at the position in the team's system are ready for MLB play.
Detroit had a need a shortstop since last year's starter, Jose Iglesias, is a free agent.
The Tigers appear to be in the midst of a rebuild, which makes Mercer an ideal short-term option to provide some leadership and hold down the fort until Detroit is ready to elevate some of its young talent.
Mercer served in a similar role with the Pirates, and he should be well equipped to do it in Detroit alongside Miguel Cabrera.