While we still don’t have robust defensive metrics, what we do have paints an acceptable picture of a given player’s defensive value. The two most commonly used defensive metrics are UZR (ultimate zone rating) and DRS (defensive runs saved), both of which are scaled to how many runs a defender saves above an average fielder. There are methodological differences between the two, all of which have been talked about exhaustively, but both can be interpreted in the same manner. Scores of +5 are considered above average; +10 is considered top-tier; +15 is for the gold glovers.
As a caution, remember that defense is quite variable, and one to three seasons of data is necessary to make a decent conclusion of one’s true worth on defense. As such, using Ian Happ’s 122 innings to extrapolate his defensive capability is a little silly, but it’s all we have right now.
With that in mind, Happ’s DRS is +1 and his UZR/150 is -4.8 when playing centerfield. This means that one system believes Happ is about average while the other thinks he’s below average. UZR, however, actually has been somewhat favorable to Happ’s range, but docks him heavily on lack of arm strength.