La Russa had been undecided between Weaver and rookie right-hander Anthony Reyes, who beat the Tigers in Game 1 on Saturday in Comerica Park. But La Russa preferred Weaver, a veteran right-hander who lost to the Tigers in Game 2 by allowing three runs in five innings, but will be pitching on his normal four days of rest.
"It will be his natural day, I think," La Russa said. "The way he's pitched for us makes sense to go, and we keep Anthony ready for whatever we need."
Weaver, who was acquired from the Los Angeles Angels of Anaheim on July 5, was 5-4 with a 5.18 ERA in 15 starts for the Cardinals during the regular season. He is 2-2 with a 2.94 ERA in four starts so far in the playoffs.
Now he could bring a World Series title back to St. Louis for the first time since 1982. With their 5-4 victory over the Tigers on Thursday, the Cardinals are now up 3-1 and one win from drinking some serious champagne.
There was a time earlier this season when it seemed a long shot that Weaver would be sipping champagne or that a manager would be foolish enough to alter his rotation so that Weaver could get a chance to pitch in such a game.
He was 3-10 with a 6.29 ERA in 16 starts for the Angels and lost his spot in the rotation to his younger brother Jered. But things changed for the better when he was traded to the Cardinals.
"This has been a crazy year, there's no doubt about that," Weaver said. "But once I had come to St. Louis, one of my major thought processes was everything kind of happens for a reason, and it continues to work and who knows, maybe we'll end up in postseason and have a chance to erase everything that happened prior.
"And once we got to the postseason, I mean that just kind of brought everything to fruition with me and everything has happened for a reason. And to have the opportunity to pitch in the postseason and help this team win some games, it's been a dream come true."
Reyes held the Tigers to two runs on four hits over eight innings in a 7-2 victory in Game 1 and La Russa has said there is still a possibility he could get one more start before the Series is over if the Cardinals don't wrap things up on Friday.
Much depends on the weather. La Russa may need a starter for Game 6 on Saturday in Detroit, and the options would be either Reyes or Chris Carpenter, who would be pitching on just three days' rest for the first time in his career.
But rain is still a possibility for Friday. That could push Game 6 back to Sunday and allow Carpenter to pitch on his regular four days' rest.
If that happens, Reyes could be a possibility for Game 7 on Monday, if necessary, unless the Cardinals decide to bring back Jeff Suppan on three days' rest. He has done that just once in his career.
"We'll be consistent," La Russa said before Game 4. "We know we're going to play two more games, and then see how our staff shakes out."
So that leaves Reyes to sit and wait until needed.
"There's so much uncertainty about how long the series is going to go, with the weather, what days," La Russa said. "He's going to throw a bullpen today and we'll see how long this thing goes. He's not going to start in the St. Louis portion, but there's a chance he would start in Detroit. So we're going to get him ready for that."
Everything, of course, is moot if Weaver takes care of business on Friday night at Busch Stadium.
"I've always been a day-to-day person, so I try not to think about anything I can't control," Reyes said. "I always go at it like I'm going to pitch, so I just do everything I was going to do before, like I am going to pitch and if it's not my time, I just try to prepare for the next one."
T.R. Sullivan is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.