PITTSBURGH -- Mitch Keller's up and down 2021 season moved into its third month without any signs of straightening out and trending up yet.

That’s not to say it won’t or can’t, but Keller posted his second consecutive start of fewer than three innings on Thursday, when he threw 72 pitches in 2 1/3 innings against the Dodgers. The outing raised his ERA to 7.04, and unlike 2019, when he had a 7.13 ERA but a 2.76 expected ERA, the expectations are less favorable this season (5.56).

Keller is staying composed on the mound and in the Zoom room, but it’s clear the frustration has begun to set in.

“I'm probably the most pissed-off guy in this locker room,” Keller said on Thursday.

“The fact that he’s frustrated, I appreciate that,” manager Derek Shelton said. “He shouldn’t be happy with the fastball command or the situation that we’re running into. We will talk about it. It will be in a very direct and a very positive tone."

What’s going wrong for Keller and what are the options for him in the near term? Let’s look at some of the numbers that may be leading to head-scratching results:

Keller’s four-seam fastball has velocity, as he’s able to sit in the mid-90s and touch 98 mph. That kind of heater tends to play in the big leagues, but it’s not been up to par in Keller’s case in terms of production against it, and that’s been largely true even with a sharp drop in weighted on-base average from his rookie season to this season.

Of 144 pitchers to throw 750 four-seamers between 2019-2021 entering Friday, Keller has the eighth-highest weighted on-base average (wOBA) off his four-seamer (.410). It was at .499 in ‘19, but has dropped to .366 this season. That’s mainly due to a drop in batting average against, but the slugging mark against of .471 has stayed fairly high, as 13 of the 25 hits off the four-seamer -- which is slightly below league average in vertical and horizontal movement -- have gone for extra bases.

Since Keller relies on his four-seam fastball so much, using it 57.2% of the time on average this season, it’s important that he locate it as well. However, among pitchers who have thrown 750 fastballs in 2021, he has the fourth-worst rate of four-seamers for balls (not including intentional balls) at 21.1%.

The whiff rate on Keller’s slider has been lower than past seasons, but not alarmingly low. However, he’s left 24.4% of his sliders over the heart of the plate, as defined by Baseball Savant, and they’ve gotten hit (.425 wOBA and a .420 expected wOBA). He’s middle of the pack in whiff rate at 31.4%, which ranks 52nd out of 80 pitchers with 100 swings induced on a slider in 2021.

Batters are squaring up his curveball like never before. Keller has the lowest whiff rate this season of any MLB pitcher with 50 swings induced on a curveball at 15.4%, which is 7% back of the next closest pitcher (Jameson Taillon).

In short, his fastball is fast, but fairly straight and not being commanded well, per his own admission and the data. His slider is finding the heart of the plate a quarter of the time, and his second offspeed pitch is being hit at a higher rate than ever before in his MLB career.

So what should the Pirates do with Keller right now? Shelton initially ruled out the possibility of a Minor League option on Thursday, saying those conversations would need to take place if that were on the table. Two days later, the club optioned Keller to Triple-A.

“We have complete faith in Mitch Keller, and Mitch Keller’s going to be a good Major League pitcher,” Shelton said.

Keller, Shelton and Cherington all trust that the work the right-hander and former No. 1 prospect in the organization is doing in the background is pointed toward a good outcome. It’s just been a bumpy road so far for Keller, who has still yet to pitch 30 Major League games and now will work back from the Minors.

"I think we have a good plan,” Keller said. “It's just me going out and executing.”