The Nationals actually have three center fielders with A Potential in Brian Goodwin, Steven Piedra and Robles. At only 20 years old, and depending on what happens with Bryce Harper, Robles could find himself starting in the Nationals outfield sooner than later. He already has great Speed (79), and good contact numbers (53 Con vs R, 57 Con vs L) that should land him near the top of Buy MLB18 Stubs the Nationals lineup as he progresses. 

Versatility is the name of the game with Happ. He is essentially the evolution of Ben Zobrist, who is also on this Cubs team. He will start in centerfield for the Cubs, but could easily start at any outfield position if you look to bring someone else in. He has good Contact numbers (55 vs R, 58 vs L) and some pretty good Power as well (82 vs R, 67 vs L) and his Speed (74) makes him the kind of super-utility player any team could use.