"It's amazing that he's been able to do what he's doing," Matheny added. "He kept saying he kind of felt weak. We ran a number of tests, [and] everything was coming back negative. There was one day where he thought he had been exposed to the sun too much. He just was feeling weak and kind of had some chills, and it wasn't necessarily anything that we could point towards. ... Hopefully he can get past this, and we can get him some rest."
Matheny said the weakness dated to June 16, the day before the club opened a three-game series against the Rangers. Siegrist has made six appearances since then and allowed one run on two hits.
The Cardinals have given Siegrist the option of returning to his Florida home if wants to rest there. He did come to the ballpark on Friday before being sent away.
The absence of Siegrist further complicates the Cardinals' bullpen, which is already in flux after Trevor Rosenthal's recent removal from the closer's role. Siegrist was one of three relievers Matheny said he would consider in save opportunities. Siegrist has a 2.78 ERA in 35 appearances this season.
Matheny noted that Rosenthal has worked himself back into being a late-inning option -- along with Seung Hwan Oh and Jonathan Broxton -- after a pair of encouraging appearances this week. Tyler Lyons becomes the team's lone left-handed reliever.
Tuivailala, named to the Triple-A All-Star Game earlier this week, was scheduled to arrive at Busch Stadium around game time on Friday. He posted a 5.06 ERA in 29 games with Memphis, converted 13 of 16 save opportunities and struck out 48 over 32 innings. Tuivailala made 14 big league appearances in 2015. This is his first callup of the season.
"He's had some really good outings; he's had some times where he has gotten himself into trouble and pitched himself out," Matheny said. "He has times where he has been put in there in tough situations and has been able to improve."
• Outfielder Randal Grichuk blasted two homers in leading Memphis to a 5-2 win on Thursday. Grichuk has four homers in 12 Triple-A games and is hitting .293/.302/.634.
• Matheny spoke with Joe Torre of the Commissioner's Office on Friday regarding his sixth-inning ejection a day earlier. Matheny's complaint was that it took too long for Royals manager Ned Yost to alert the umpires that he wished to challenge a call. According to Matheny, Torre said he understood why Matheny was upset about the call and that teams/umpires can do a better job enforcing a time limit on reviews.
Jenifer Langosch has covered the Cardinals for MLB.com since 2012, and previously covered the Pirates from 2007-11. Read her blog, follow her on Twitter, like her Facebook page and listen to her podcast. This story was not subject to the approval of Major League Baseball or its clubs.