The pitcher -- who had led the Southern League with a 1.30 ERA on May 3 -- all of a sudden was 0-3 with a 9.15 ERA over five starts in the third month of the season.
Now he seems to have found the pitch again, and his Double-A opponents are once again paying the price.
Mariners' No. 2 prospect allowed only two hits over six scoreless innings Thursday as Jackson defeated Montgomery, 5-2. The 6-foot-4 right-hander also struck out eight, his highest total since fanning a season-high nine on June 3.
Thursday's performance marked the first time he hadn't allowed a run in an outing since May 3. That start rounded out a run in which he had tossed shutouts in three of his first five starts at his then-new level.
But when June hit hard, Walker admitted he was slightly shaken by his first real taste of adversity in the Minor Leagues.
"I had been working a lot," he said. "June didn't really go well for me, and so I had to go back and work on some things. I was just trying to build confidence and get back on track."
The biggest key there was indeed the offspeed stuff. Walker owns a fastball in the mid- to high-90's that was more than enough to carry him to an 2.89 ERA last year for Class A Clinton. His curveball has also been considered a plus while a changeup was in development. But when he lost confidence in the latter two pitches, his effectiveness also went away.
"The curveball just wasn't working," said MLB.com's No. 13 prospect. "So when that happened, all I would want to throw is my fastball. The hitters, they're better here at Double-A, so they took advantage. ... It got to the point where every time my catcher called for an off-speed pitch, I would worry. When you're thinking like that, it's going to be a ball every time."
Thanks to the extra attention to the slower, more deceptive pitches, something "clicked" around the turn of the calendar to July, Walker believes. Ever since, his outings have grown increasingly better. Including Thursday's gem, he is 3-1 with a 3.13 ERA in four July outings and has tossed quality starts in each of his past two starts.
But the midseason stumbles also have provided the 19-year-old with valuable lessons about what it takes to dig yourself out of a hole in the Minor Leagues.
"You just have to trust yourself," he said. "Everyone's going to have struggles at some point when they're a pro. You just have to work to get yourself out. So whether it's busting my butt in the gym or working hard in bullpens, I did whatever I had to do to get back on track. I think I'm there now."
Mariners' No. 7 prospect Brad Miller hit the first Double-A home run of his career -- a two-run shot in the fourth. The shortstop, playing in his third game for the Generals, finished 2-for-4. Leadoff hitter Nate Tenbrink also smacked a three-run homer to complete the squad's scoring.
Rays' top prospect Hak-Ju Lee finished 1-for-4 with a double for the losing Biscuits. In doing so, he extended his on-base streak to a franchise-record 38 games.