Diaz, making his first appearance since he suffered a forearm strain in late June, entered in the third inning, after St. Louis had roughed up Padres starter Travis Wood for seven runs. The 22-year-old rookie promptly stranded a pair, and he would set career highs with four innings and six strikeouts.
"That guy we saw today has the potential to start in the big leagues," said Padres manager Andy Green. "We've always believed in that potential. Now, he's moving in the right direction."
Diaz left his outing in Chicago on June 21 with tightness in his right forearm -- a setback that cost him 2 1/2 months of his rookie season. That downtime wasn't for naught, however. While he recovered at the team's Spring Training complex in Peoria, Ariz., Diaz worked extensively with the Padres' staff to incorporate his lower half into his delivery.
"It's something I was focusing on," Diaz said. "When I wasn't throwing, I did a lot of work on my legs. Tonight I felt like I might have stood up on a few pitches early on, but I was able to make that adjustment and use my legs."
The future is still a bit uncertain for Diaz, whose best chance to make the Opening Day roster next spring might be as a reliever. That said, the organization views Diaz as a starter in the long term. He feels the same way.
"I have to build up that endurance, and then always throw first-pitch strikes," Diaz said, when asked to pinpoint where he needs to make strides as a starter.
In 25 appearances this season (including three starts), Diaz owns a 6.06 ERA and a 1.49 WHIP. Those struggles were probably to be expected for a young hurler who had never thrown a pitch above Class A until Opening Day.
But he's shown more than a few flashes of potential this season -- with Tuesday serving as a prime example. He punctuated his outing with consecutive strikeouts in the sixth -- first with a tight curveball to catch Harrison Bader looking, then with an elevated 97-mph heater to get Randal Grichuk swinging.
"He could pitch himself into our plans in the bullpen, and he could pitch himself into our plans in the rotation," said Green, when asked where Diaz fits in 2018. "All those things are possible. He's going to have to earn his spot onto the Major League club next year if he's going to be with us. ... He has some ground to make up. But everything will be open for him as far as competition goes coming into Spring Training."