In Gonzales, the Mariners land a solid left-handed starter who has done well in his return from Tommy John surgery in 2017. He was buried down the Cardinals' depth chart, but should get better opportunities to start in MLB with Seattle moving forward.

The Cards already boast five quality starters, and should get back top pitching prospect Alex Reyes at some point in 2018, so they could afford to deal Gonzales at this time. In O'Neill, St. Louis adds a premier power hitter who could be starting in left field in 2018.

Both Gonzales and O'Neill could see their fantasy stock rise following this swap, even though neither may make an impact until 2018. Both players are decent keepers, though O'Neill has more long-range value overall.