I think the Nats held him back this spring was so he could pitch through September.
Could be possible but I think they're just watching his innings, like Giolito. He's actually behind in terms of experience than Giolito. Gio has pitched 147 innings as a pro, with a high of 98. Lopez has 116 pro innings, with a high of 83. So his innings are going to be carefully monitored.
With that said, it's not out of the realm of possibility he helps the pen in September. If you get him to about 100 innings as a starter, then make him a reliever, he gets an extra 15-20 in the pen. But, I don't think the team will rush him or risk any developmental problems by messing with him or rushing. He's got big time upside and some people don't take well in changing roles. And while he does seem to have electric stuff, he doesn't seem to finish guys off at a high rate. He already hits 98 now, so it's doubtful he picks up 4/5mph in shorter stints to give him an extra boost. I don't think his stuff and K rate will improve at the big league level just yet.
I'm for it if he can dominate this year and gets a fair amount of experience in AA, but I just think the team will be more patient.