According to NFL Network's Ian Rapoport, the Niners will send a 2019 fifth-round draft pick to the Lions in exchange for the former first-round pick.
Detroit selected Tomlinson with the No. 28 overall pick in the 2015 NFL draft out of Duke, and he primarily served as a starter in his two seasons with the team.
Tomlinson was active for all 16 games in both 2015 and 2016, making 14 starts as a rookie and 10 starts last season.
He became expendable for the Lions after they signed former Green Bay Packers offensive lineman T.J. Lang.
With Lang and Graham Glasgow set to start at guard, Tomlinson would have only been an insurance policy on the interior offensive line for the Lions in 2017.
San Francisco has a bit more uncertainty on its offensive line. Right guard Joshua Garnett is currently out with a knee injury, leaving veterans Brandon Fusco and Zane Beadles in the mix for playing time at guard.
Tomlinson has far more upside, and surrendering only a future fifth-round pick is a worthwhile gamble in exchange for a player who boasts first-round talent.