In recent years, the month of August has been a rough one for veteran reliever Trevor Rosenthal.

In August 2017, he underwent Tommy John surgery, and was still in the midst of his recovery and without a team one year later. Then, last August, he was released for the second time in the 2019 campaign. But it’s a different story in 2020.

With the Trade Deadline looming on Aug. 31, the Royals right-hander is one of the hottest names on the market, and MLB.com’s Mark Feinsand recently put him first on the list of players most likely to be dealt.

Part of the widespread interest in the impending free agent is due to the dearth of teams in selling mode, given this year’s expanded postseason field. But Rosenthal’s performance also deserves recognition, as he has recorded a 1.59 ERA with 15 strikeouts, five walks and a 1.06 WHIP in 11 1/3 innings. He has notched six saves in six chances, too.

The 30-year-old's success this season is especially remarkable considering the extent of his struggles in 2019, when he had a 13.50 ERA over 22 appearances and was released by both the Nationals and Tigers. The Missouri native joined the Royals on a Minor League deal in January.

Let’s take a look at what happened last season, what Rosenthal is doing differently in 2020 and whether he is truly back to being the dominant fireman he was for the Cardinals in ’14-15. (The stats below are as of Sunday.)

What went wrong in 2019?

In a word, everything.

Prior to undergoing Tommy John surgery in 2017, Rosenthal had a 2.99 ERA with a 12.0 K/9 mark in six seasons for St. Louis. He recorded 93 saves over 2014-15, including a team record 48 in '15, his lone All-Star campaign.

Although Rosenthal had missed more than a year, the Nationals signed him to a one-year, $7 million deal and were counting on him to help fortify the back end of their bullpen as a setup man for closer Sean Doolittle.

His return season quickly went off the rails, however, as he didn’t get a single out while allowing seven runs on four hits and four walks in his first four appearances. He wasn’t able to get on track after that, and had a 22.74 ERA with five K’s and 15 walks in 6 1/3 innings when Washington released him in June. 

Rosenthal subsequently signed a Minor League deal with Detroit and found some early success after joining the team, yielding just two earned runs in his first eight innings, but he was released again after back-to-back rocky outings. Between the two clubs, Rosenthal walked more than 30% of the batters he faced.

What’s different in 2020?

For Rosenthal, it’s all about the fastball. A lack of velocity has never been an issue for him, and it wasn’t last year either, as he averaged 98 mph with his four-seamer.

His command of the pitch was sorely lacking, however. Rosenthal threw only 41% of his four-seamers in the strike zone, and he often missed by a wide margin.

Of his 283 total four-seamers, 16.6% came in the “waste” zone, the highest rate in MLB by nearly three percentage points. That made the pitch easy to lay off. Last season, hitters chased Rosenthal’s fastball only 16.8% of the time when he threw it out of the strike zone.

This year, while averaging 98.1 mph, Rosenthal has fired 54.2% of his four-seamers in the strike zone, and he’s recorded a chase rate of 24.1% on fastballs out of the zone. Only 5.9% of his fastballs have come in the waste zone.

Rosenthal’s inability to locate his fastball consistently last year meant that he was often pitching behind in the count, which allowed hitters to more easily spit on his slider and hunt for fastballs in the zone. Now, Rosenthal is not only throwing his fastball for strikes and getting ahead of hitters but also using it to put them away.

Rosenthal has thrown 31 two-strike fastballs in 2020 and collected nine strikeouts, giving him a putaway rate of 29% with the pitch -- tied for the 13th-highest in MLB. Rosenthal’s putaway rate on fastballs in 2019 was only 12.2%.

Meanwhile, Rosenthal’s swing rate on his slider is up to 54.3% (49.5% in 2019), and his whiff rate on the pitch has jumped to 32% (28.6% in ’19).

Royals skipper Mike Matheny, who also managed Rosenthal in St. Louis, pointed to Rosenthal’s improved mechanics as the reason for his success this season.

“I think he just got back to what he knows works for him,” Matheny told the Kansas City Star’s Jesse Newell. “Everything just looked timed-up right. I think he was allowing himself biomechanically to have that range to stay square to the plate. He was falling off a lot last year. I think a lot of it had to do with, maybe, a limited range [after Tommy John surgery]. To me he looks extremely similar to what I’ve seen him when he’s been at his best.”

Rosenthal’s average horizontal release point has shifted nearly nine inches toward first base since last season, while his vertical release point is up an inch and a half, which lends some support to Matheny’s theory that the righty is staying more square to the plate, instead of letting his body fall toward first base while dropping his arm slot toward third. Looking at his average release points from other years, Rosenthal is now closer to what worked for him in the past.

Perhaps as a product of his mechanical improvements, Rosenthal has managed to bump his average four-seam fastball spin rate to 2,423 rpm, up from 2,348 in 2019. Fastballs with high spin can defy gravity for longer so that they appear to have a rising effect, leading to more whiffs and fly balls.

In 2020, Rosenthal's four-seamer has had two more inches (16% above average) of "rise" than similar four-seamers at his velocity, an increase from 0.9 inches (7% above average) in '19.

In another positive development, Rosenthal has brought back the changeup he all but abandoned last season, when he threw it only 14 times. The right-hander already has thrown 17 changeups in 2020, with 16 of them coming against left-handed batters.

Rosenthal used his changeup even more than his slider at times in the past, and it proved to be an effective secondary offering. In fact, Rosenthal’s changeup has yielded one of the lowest xwOBA marks in Statcast history (since 2015).

Lowest xwOBA on changeups, since 2015
Min. 400 changeups thrown
1. Roberto Osuna -- .151
2. AJ Ramos -- .176
3. Stephen Strasburg -- .185
4. Trevor Rosenthal -- .191
5. Fernando Rodney -- .199

Left-handed batters pummeled Rosenthal’s fastball in 2019 (.309 xBA, .538 xSLG), but they have recorded a lowly .129 xBA and .141 xSLG against the pitch this season. Coupled with the improved command, the increase in his changeup usage helps explain why his fastball has been so much more effective against lefties.

Overall, Rosenthal ranks in the 96th percentile in xwOBA and xBA and the 93rd percentile in xSLG this season, which suggests his hot start is built to last. But will it?

Based on what the flamethrowing righty has shown so far in 2020, there are likely going to be a lot of teams at the Deadline willing to take a chance that the answer is yes.