It's hard to get excited about middle relief prospects. I know that I certainly poked my share of fun at the Rivero signing when it was made. He got $3.1 million, and Brett had this to say:
While he may prove a valuable piece of the bullpen in future years, this is simply about accumulating talented pitching, and paying handsomely to do it (because the routes are limited these days). Even if he merely contributes as a decent middle reliever for a few years, it was probably worth the investment.
I said something along the lines of "if you have an opportunity to pay a 25-year old who hasn't played in America $3.1 million to be a middle reliever, you have to take it." Nice snark.
Rivero was as elite as you'd expect a 25 year old player to be. In 18.1 innings at Kane County, he struck out 33.7% of his batters (though he walked 10.8%). This is a small sample size (it all is, he pitched 30.1 innings this season), but gives you an idea of his skillset. He also allowed 4 HR in his limited time at the KC. That was the primary driver of his 4.62 FIP at that level; I'm not a fan of xFIP (and you can't really get it for minor league teams), but I'm positive that Rivero's true HR/9 isn't 1.96, so that FIP doesn't mean much to me. He saw 10 outs of Daytona and 26 at Tennessee, and was very dominant at each stop. All told, he recorded 91 outs, 45 of them by strikeout. He also walked 8.8% of his batters.
When Armando signed, Ben Badler had this to say about him:
Rivero's best pitch is his fastball, which sits in the low-90s and peaks at 96 mph. He doesn't have a plus secondary pitch but he has a solid splitter with late tumble. He'll drop to a low three-quarters arm slot, which may be why he has trouble throwing a reliable breaking ball. Some scouts have said Rivero throws a curveball and a slider, while others think he's just manipulating the same pitch. His low slow makes it difficult for him to stay on top of the ball, giving his breaking ball more side to side action. He also has a slight hook in his arm action that affects his command.
That sounds like the prototypical middle reliever to me. He's 6'3", 180 pounds, so he's probably wiry. It's really hard to find any actual video on him, so the scouting is pretty light. All I can go on is his spurious sample size at his 3 stops this year.
Rivero turns 26 on Februrary 1st, so he's going to be old for any level except the majors. He's not on the 40-man, but he's going to get every chance to make the big-league club out of spring training, and it'd be a disappointment if he started anywhere lower than Iowa.