I'm pretty sure I had seen a stat that indicated when he was called up he had the highest defensive runs saved of any outfielder in the minor leagues at that point. Not just Eastern league, or AA. the entire full-season minor leagues from AAA all to Low-A.
He has 5 defensive runs saved in just 144 innings. (16 games)
To put in perspective looking at all MLB CFers
#1) Kevin Kiermaier: 13 DRS (714.1 inn)
#2) Daulton Vasho: 12 DRS (321.1 inn)
#3) Kyle Isbel: 9 DRS (470.1 inn)
#4) Brenton Doyle: 7 DRS (638.0 inn)
#5) Alek Thomas: 7 DRS (582.0 inn)
#6) Harrison Bader: 6 DRS (533.0 inn)
#7) Johan Rojas: 5 DRS (144.0 inn)
#8) Jake Myers: 5 DRS (672.1 inn)
#9) Luis Robert: 5 DRS (915.0 inn)
So he's in 7th despite having considerably fewer time than almost anyone else in front of him to accrue DRS. 144 innings = 16 games.
He's recording about 0.35 DRS per inning. Dalton Vasho who is at #2 with 12 DRS has acrrued about 0.37 per inning and the only CFer with a higher pace than Rojas.
Although UZR seems to favor Rojas as he's at 26.0 UZR/150 which is #1 among all CFers with at least 100 innings.
UZR/150
#1) Johan Rojas 26.0
#2) Daulton Vasho 21.3
#3) Brenton Doyle 20.2
so UZR/150 projects if he played 150 games*9 innings in CF he would save 26 runs over the course of a season.