"He's tough as nails, that kid. It's been a nice a year for him," Reds manager Bryan Price said at the end of the season. "Talk about a guy that's cemented himself into an important role in our bullpen. He's certainly done that."
Peralta, 26, completed his first full big-league season by going 3-4 with a 3.76 ERA while leading all Major League rookies with 69 appearances, placing him second on the club in games pitched. In 64 2/3 innings, he allowed 53 hits and 24 walks while striking out 57. Lefty hitters batted .214 against him, but Peralta was also effective vs. right-handers, who batted .237.
One of the last relievers to secure a spot in Spring Training, Peralta eventually leapfrogged over Tony Cingrani while the veteran was either injured or struggling. Cingrani eventually became expendable and was traded to the Dodgers on July 31.
Peralta went home for the offseason pleased that he showed he belonged in the Major Leagues and had the ability to also pull his weight.
"It feels way better knowing I'm in there but you can't get too comfortable," Peralta said via translator Julio Morillo. "I told them I am going to go back to the Dominican Republic and work hard. Now it's more mental than physical but I have to keep working as I've worked my whole career."
A former starter in the Minor Leagues, Peralta always had a plus fastball in the 94-97 mph range. But as a reliever, including during a 2016 September call-up, he struggled with the command of that pitch and lacked a credible changeup and slider. In '17, he made the needed corrections by developing his secondary pitches with the help of the coaching staff.
According to Statcast™ data, the slider became Peralta's most-used pitch, at just over 27 percent. It had a whiff rate of 21 percent, second to his changeup, which induced swings-and-misses 28 percent of the time. Peralta used his four-seam fastball and sinker 26 percent of the time each. He used his changeup 19 percent.
"The big part of my success this year is that my pitches are way better and my command is way better," Peralta said. "If you compared me to the pitcher I was last year, it's a really big difference."
Another improvement Peralta made was mental, by being able to control his emotions on the mound.
"Last year if I gave up hit, I would throw harder," Peralta said. "I was a thrower and not a pitcher last year. It doesn't matter if I give up a base hit, I have to keep doing my job."
Price noticed the differences in Peralta early on in camp, but didn't totally expect the type of the season he would end up having.
"Not a total surprise, but I don't think you can anticipate a rookie performing to that degree and doing it with such confidence," Price said. "At no point, has he looked like he was scared, intimidated, reluctant and some of the things you see in young players."