Ranking the Rockies: No. 16 Jairo Diaz began to harness his big fastball in 2015

When he was acquired from the Los Angeles Angels in a trade for Josh Rutledge last December, Colorado Rockies reliever Jairo Diaz came with a reputation of having a big-time fastball but not much ability to command it.

In his first season with the Rockies, Diaz began to get the fastball under control and started to mold himself into more of a pitcher than a thrower.

Among the youngest relievers in the Rockies bullpen at age 24, Diaz made his Colorado debut on August 23 against the Mets, pitching a scoreless inning with a walk and a strikeout. It was the first of 21 appearances Diaz made for the Rockies in 2015, posting a 2.37 ERA and 1.16 FIP in 19 innings, walking six and striking out 18.

The walk number showed a significant improvement from his time at Triple-A Albuquerque this season, where he walked 37 in just 55 innings. As importantly for someone pitching half his games at Coors Field, Diaz induced ground balls at a rate of 56.6 percent and surrendered just two home runs for the Rockies in 2015.

Diaz's effectiveness eventually earned him a steady role in the eighth inning for the Rockies through most of September setting up closer John Axford, a role Rockies manager Walt Weiss said he can see Diaz continuing in the future.

"He got a cup of coffee last year with the Angels," Weiss said. "But we were excited about getting him in a trade and the power that he brings and to see it play out here in major league games, late in major league games, has been very encouraging."

The power Weiss speaks of is no joke. Diaz's fastball, which he threw more than half the time in his stint with the Rockies, averaged 97.4 mph and touched the 100 mph mark at times, which puts his velocity in a class with All-Stars Craig Kimbrel, Trevor Rosenthal and Kelvin Herrera. He augmented that heater with a slider that sat around 90 mph that he threw roughly 40 percent of the time.

What was perhaps most notable about Diaz's performance in 2015 was that he was worth nearly a full win, accumulating 0.7 WAR according to Baseball-Reference, and ranked in the top 20 among Rockies in WAR despite pitching fewer than 20 innings.

If he can keep the walks down and the velocity up, Diaz could prove to be a key late-inning reliever, perhaps even a closer, for years to come and make the Rutledge trade look like a steal for Jeff Bridich.