"They went and did what we asked them to, which was go done there and perform and be the best two options when we need options and they've done that," Reds manager Bryan Price said. "They come back up with a chance to get reacclimated here and help us out."
Diaz and Sampson both appeared in Tuesday's 8-7, 12-inning loss to the Indians. Diaz allowed a run in 1 1/3 innings, while Sampson surrendered Francisco Lindor's 12th-inning home run to take the loss.
Both pitchers were sent to Minors with instructions to improve their command of the strike zone. From Louisville, manager Delino DeShields and pitching coach Ted Power provided positive reports.
"Those two needed to go down and cluster pitches and become reliable with the command because the stuff is already there," Price said.
Diaz, 32, has a 5.63 ERA with five walks, three home runs and five strikeouts in eight innings over nine games with the Reds this season. At Louisville, he worked 10 scoreless innings over 10 games with eight hits, two walks and 15 strikeouts.
In his earlier big league stint, Diaz showed slightly decreased fastball velocity but no drop in his changeup speed. That made it harder to deceive hitters.
"Everything was working right down there," Diaz said of Louisville. "I had fun. My fastball was working good. I worked on my changeup because I was throwing my changeup a little harder here. I went there and threw it a little bit softer and my slider is working good."
Sampson, 25, has pitched in three big league games this season and allowed five earned runs in four innings. To get him back on the 40-man roster, the Reds transferred Homer Bailey to the 60-day disabled list.
"I just went down there with a game plan," Sampson said. "A mindset that I'd do everything to get back here and that was making sure I was controlling my fastball and throwing my slider and curveball for strikes and bounce one when I needed to. Really, just mentally getting back and prepared and focused and being able to turn the page if something happens."
Cincinnati's bullpen has been taxed for 13 1/3 innings of work over the last three games -- two of which were blowout losses to the Indians on Monday and Tuesday. The Reds have the highest bullpen ERA (6.43) in the Majors and leads in walks allowed, WHIP and home runs allowed.
"This is the way the system is supposed to work, when you have guys that struggle, you utilize your farm system to get back on top of their game," Price said.