Reyes' arm is certainly fresh, so it's not a matter of watching his workload. After serving a 50-game suspension for marijuana use, he has thrown just 65 1/3 innings this year. And in that span, he's struck out 93, or 12.8-per-nine innings. At the same time, he's walked 32, a rate of 4.4-per-nine. That's actually a touch under his career rate of 4.6, but it's easy to see what the issue has been for Reyes as a starter: command.
The stuff has never been in question for Reyes since he signed out of the Dominican Republic (after returning there following growing up in New Jersey) in December 2012. He has one of the most electric fastballs in the game, one that sits in the mid-to-upper 90s and touches triple digits fairly regularly, as he did three times in his 1 2/3 innings of work in the Futures Game that saw him strikeout four. While only about 13 percent of all Major League fastballs are 96 or higher, Reyes -- albeit in a shorter stint -- didn't throw any of his 20 Futures Game fastballs below 96.4 mph. He also threw it with above-average spin (2,358 rpm), which means he throws his fastball not only with velocity, but also with good life.
Reyes isn't strictly a fastball pitcher, either. He has one of the best power curveballs in the Minors, a true swing-and-miss pitch. The combination of Reyes' heater and breaking ball probably would be enough to get big league hitters out in a relief role, but that would be underselling his changeup. Yes, it's his third pitch, but it's improved, and he has always shown an understanding that he needs one to make him a more complete pitcher. In the Futures Game, Reyes featured a changeup that averaged 84.9 mph. That's nearly a 14-mph differentiation from his fastball, an almost unheard of separation, especially for a power pitcher.
Just because Reyes is serving in a bullpen role now does not mean the Cardinals are giving up on him as a future front-line starter. But he might want to be careful. If he's too good in shorter stints -- something that is very likely to happen -- it might make it tough for him to get back to a rotation. Just ask Trevor Rosenthal, who was a pretty good starting pitching prospect when he was called up to help St. Louis' bullpen and has never started a game since. If someone said Reyes would one day be closing games for the Cards instead of starting them, it would be easy to believe.
The 23-year-old Weaver, on the other hand, has proven that he has long-term starter potential, though that hasn't always been the case. The 2014 first-round pick out of Florida State has had plenty of fastball as a pro, sitting 94 mph consistently after a velocity dip during his junior year of college, and he can reach back for a bit more when needed while maintaining the mph deep into starts. Weaver couples that with a plus changeup, thrown with outstanding sink.
Weaver will throw a curve and a slider, and while both are fringy, they both have the chance to be Major League average, and one can be better than the other on any given day. The combination has allowed him to miss plenty of bats this season (10.0/9), and unlike Reyes, he never hurts himself with walks, with a 1.6 per nine rate in his Minor League career (1.3 in 2016).
It should be noted that neither Reyes nor Weaver were players who needed to be protected on the 40-man roster this offseason to keep from being eligible for Rule 5 Draft. As a result, they shouldn't be seen as short-term insurance and could stick around awhile if needed. At the very least, it seems apparent that when Weaver comes up, it will not be Reyes who gets sent down to make room.