3. Trevor Siemian
Siemian made four starts for the New Orleans Saints last season, with 11 touchdowns and three interceptions over six total appearances. Before that, he made one start (for the New York Jets in 2019) over the prior three seasons.
Siemian has had the most substantial action in his career with the post-Peyton Manning (and Super Bowl 50 win) Denver Broncos. He made 24 starts over the 2016 and 2017 seasons, as first-round pick Paxton Lynch proved he was a bust right off the bat. He went 13-11 in those starts, with a positive touchdown-to-interception ratio (30:24), a completion rate a shade above 59 percent (59.3) and a passer rating a litle shy of 80 (79.9) over 25 total games over that span. Perfectly backup-caliber quarterback stats.
Siemian would be a solid upgrade over Boyle, but shy of a serious candidate to beat Goff out for the starting job. That looks like the sweet spot for the Lions to hit in their search for a new backup quarterback.