Stewart exited the Pirates' 4-3 win over the D-backs on Monday at PNC Park during the seventh inning after legging out a go-ahead, two-run triple to left-center field. Stewart said he felt a pull in his lower left hamstring as he rounded second base, but his initial tests after the game revealed "nothing major."
Stewart said his left leg still felt sore and stiff when he woke up Tuesday morning. He is expected to undergo an MRI exam on Wednesday morning. Rather than play without a backup catcher behind Francisco Cervelli, the Bucs sent Stewart to the disabled list and recalled Diaz for the second time this season.
"Tried to test it out and it didn't go well, so we made the smart call and got somebody else here," Stewart said. "I don't think any significant damage happened, but we won't see that until we get an actual MRI and see the full scope of everything."
Diaz, 26, went 2-for-9 with a double in four games for Pittsburgh in early May. He has yet to experience an extended stint in the Majors, but he has been called up each of the past three seasons.
"Another opportunity that I have to play this game in the Majors," Diaz said Tuesday. "I'm excited to be here."
Diaz had a .281/.298/.388 slash line in 30 games for Indianapolis. He went 8-for-28 with a homer and two doubles in his last seven games.
"His overall game's been good," manager Clint Hurdle said. "He'll get an opportunity to play while he's here."
Stewart, 35, is hitting .225/.295/.350 in 16 games this season. Monday's triple was his second of the season and, oddly, his 11-year Major League career. Hurdle joked afterward that Stewart will be limited to two bases the rest of his career unless he hits a home run.
"I proved that the first one wasn't a fluke," Stewart added, "so I can stop at second base from now on."
Right fielder Gregory Polanco also exited Monday's game early due to a sprained right ankle, and he was wearing a walking boot after the game. He did not play in Tuesday night's 3-0 loss to the D-backs at PNC Park.
Hurdle did not have an update on Polanco, who did not take batting practice on the field Tuesday afternoon, when he met with the media before the game.
Around the horn
• Adam Frazier was out of the starting lineup for the second straight game Tuesday. Hurdle said the Pirates have seen some signs of fatigue in Frazier, who is 1-for-14 during this homestand. Frazier has started 14 of the Pirates' past 17 games and mostly played the outfield since he came off the disabled list May 12. Frazier did pinch-hit Tuesday and notched a single.
"I think we pushed him to the point that he got a little bit fatigued. Swing fatigue, run fatigue," Hurdle said. "Just giving him a little bit of a breather."
• The Pirates have an off-day on both sides of this weekend's series in New York, giving them an opportunity to rearrange their rotation or skip a starter. Hurdle said the Bucs are still considering their options.
• Jameson Taillon threw his between-starts bullpen session as scheduled Tuesday. Taillon said Monday he expects to make at least one more Minor League rehab start, though it's unclear when or where that will take place.
Adam Berry has covered the Pirates for MLB.com since 2015. Follow him on Twitter and Facebook, read his blog and listen to his podcast. This story was not subject to the approval of Major League Baseball or its clubs.