Donaldson checks off all the boxes that make players underrated: hitter in pitcher's park, great defender but without gold gloves, didn't make majors until mid 20s, not a big average hitter, wasn't an elite prospect, plays in small market, good power hitter yet has never topped 30 HR, 100 RBI or .500 SLG, etc.
He's a total stud and the four years mean we're not "mortgaging the future."