Well, change-ups aren't bad for your arm. But in any case, Greene's currently mediocre off-speed stuff is a huge risk factor. A lot of touted HS arms in the past already had a signature out pitch - Cole Hamels and the change-up, Josh Beckett and the curve, etc.
I have nothing against HS picks, I just don't see 1-1 with Greene. In pro ball he'll sit mid-90s, which is fine but nothing that special these days, and very few prospects develop elite off-speed pitches unless they already had a natural feel for it as an amateur. Plus I think Greene is a particularly bad fit for the Twins, given their pitcher development track record - for sure Greene needs a ton of development, even if you like him as a prospect.
I'm not a scout, so I don't say what the Twins should do, but passing on Greene makes complete sense.
Why would he throw 5 mph less in pro ball?