Siegrist eventually entered to hold a one-run lead (which he did), after which Matheny acknowledged that Rosenthal had to be avoided due to tightness in his throwing arm.
"Just one of those days where his arm didn't feel right and we needed to give him a day," Matheny said.
Matheny added that Rosenthal would not have been available for Sunday's game had it not been washed out, but that the Cardinals do not believe the issue to be serious. Rosenthal, who has thrown 17 2/3 consecutive scoreless innings and converted 21 of his 22 save opportunities, last threw on Saturday, when he notched a four-out save.
That has been his only appearance since last Wednesday.
"[We] hope he feels better tomorrow," Matheny said. "That's the whole point of going around talking to the guys about how they feel. If there's a day where they say, 'You know, my arm just didn't bounce back,' which is the case with ... Trevor, if something doesn't feel as good as it normally does, we have to take a step back."
Siegrist, who now has two saves on the season, opened the ninth by freezing Joe Mauer for a strikeout. He made a nifty barehanded play to notch the second out and, after hitting Torii Hunter, wrapped up the team's fourth straight win with the help of Matt Carpenter's diving catch on Kurt Suzuki's line drive.
Matheny had given Siegrist a heads-up before the game that he was a strong candidate to pitch if a save situation arose.
"I just feel like the crowd is a little bit more into it in the ninth, obviously, but as far as me preparing for an inning, it's the same as far as the way I think," said Siegrist, typically the team's go-to setup man. "It's a big spot. But [the] eighth inning [of] close games, they're not the same, but pretty close."
Siegrist has now held opponents scoreless in 29 of his 32 appearances.
"This was a good step for Kevin to take advantage of and show us he's ready for that when the time comes," Matheny said. "He came out with a little extra fire afterward, and you could see that was something he was proud of himself for."