He could be great. He could be good. He could disappoint even if he throws a magic gyro ball.
Whatever becomes of him, it's a gamble in the neighborhood of 5years, 100 million total. That's a gamble a team with a depth of pitching prospects and a lot of positional uncertainty* does not need to make.
*Honestly other than a healthy Zimmerman who do you 100% trust to be productive next year? Ok, I'll give you Morse, I don't see him suddenly becoming bad. No, I won't give you Ramos. I like him too but 100% is 100%. The other Nats at or above average at the plate last year? Espinosa (If I'm not giving you Ramos, I'm certainly not giving you Espinosa), Nix, and Hairston.