He's not historically bad out there by DRS standards.
The following players were WORSE than -4 DRS in LF last season:
Benintendi
Profar
TEOSCAR who everyone wanted to put there
Arozarena
Austin Hays
MJ Melendez
Jack Suwinski
James Wood
Nick Gordan
Austin Martin
Yelich
Bryan Reynolds who everyone wanted to trade for in a recent offseason
Yordan Alvarez
Honestly, if the Sox traded for Bryan Reynolds to play everyday in LF, most people wouldn't bat an eye. However, he's horrible at defense (per above) and his OPS the last two seasons was 790. If you gave him fulltime reps, Masa could likely hit 790 when healthy and play bad defense. I think we overstate the negative value he brings to the team because of his lack of power, handedness and size of his contract. I think he's low key a decent player who was just injured all of last season.