ST. PETERSBURG -- Every young player is going to experience some highs and lows in their first big league seasons, and Brendan McKay is definitely experiencing his first rough stretch as a member of the Rays.

McKay enjoyed some early success at the Major League level, allowing just three earned runs over 16 innings in his first three career starts. The 23-year-old left-hander, who is the top pitching prospect in the Rays’ organization per MLB Pipeline, kept bouncing between Triple-A Durham and the big league club, but he has found a more stable role in the rotation as Tampa Bay battles through injuries to Tyler Glasnow, Blake Snell and Yonny Chirinos.

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After that early success, McKay has run into some trouble over his past two starts, including Monday’s 9-3 loss to the Mariners at Tropicana Field, when he allowed seven runs (three earned) and was able to pitch only two innings.

After the game, McKay was optioned to Durham. Tampa Bay called up left-hander Hoby Milner from the Bulls.

Over his past two starts, McKay has allowed 11 runs (seven earned). Perhaps most surprising, he has walked nine hitters after walking just three in his first six starts.

The walks played a big role in Monday’s loss. Tom Murphy gave the Mariners the early lead with a first-inning three-run home run after McKay allowed back-to-back walks earlier in the inning.

“Just some of the same struggles that I’ve had in a couple of past starts,” McKay said. “Just falling behind hitters, and it’s a lot easier for hitters to hit when they can be more aggressive and free than just making a couple of quality pitches and not getting the results you want or strikes.”

McKay struggled to find consistency in the zone, while also struggling to get any swings and misses. Of the 59 pitches McKay threw, the Mariners swung and missed on just three. He also threw 37 fastballs and only 10 curveballs.

“He was really fastball heavy,” Rays manager Kevin Cash said. “I know he’s a fastball pitcher, but you have to be able to mix a little bit. The two walks didn’t help early on. It’s something that we have to help him with and get him in a better spot over the plate, and then the mechanics allow him to do that.”

While the walks were the primary culprit in the second consecutive tough outing for McKay, a tight strike zone from home-plate umpire C.B. Bucknor and some sloppy defense behind him also were factors. In the second inning, it appeared that McKay threw a fastball in the zone on a 3-2 pitch to Keon Broxton, but it was called a ball. That allowed Mallex Smith, who was attempting to steal second base on the pitch, to get in safely despite Travis d’Arnaud’s throw beating him to the bag. Instead of two outs on the pitch, the Rays got none.

“I threw the ball fairly where I wanted to,” McKay said. “They laid off a lot of good pitches, and some calls here and there were pretty tough, but they just made some pretty good swings on some of the tough pitches, too, which is tough but it’s just how it goes.”

In addition, the Rays committed two errors in the inning that allowed the Mariners to put up four runs. Willy Adames, who has played himself into the Gold Glove conversation at shortstop, committed a throwing error on a Smith grounder to start the inning. Then Eric Sogard committed an error that allowed the Mariners to score the fourth run of the game.

A two-run home run by Austin Nola capped the night for McKay and put some pressure on the Rays’ bullpen as the team used Jose Alvarado, Chaz Roe, Oliver Drake, Nick Anderson and Diego Castillo to cover six innings. After exhausting the ‘pen, the Rays had infielder Mike Brosseau pitch the ninth inning.

With Jalen Beeks, who has also struggled as of late, scheduled to pitch the bulk innings on Tuesday, the Rays are going to have their work cut out for them as they try to salvage an opportunity to secure a winning homestand.

The past two starts have served as a learning experience for McKay, but with Glasnow, Snell and Chirinos still sidelined with injuries, the Rays are probably going to need the rookie again as they look to lock up a playoff berth.

“Certainly his last two outings, a little bit of a rut. But that happens,” Cash said. “The best ones -- Charlie Morton has been in a rut before. We haven’t seen it this season, but certainly throughout his career. All the good ones go through it, and Brendan is going to be really good, and he’s going to be a really good pitcher. He just has to work through some stuff right now.”