Manager Joe Torre said he chose Belliard, who hit .351 with five homers and 17 RBIs in 24 games with the Dodgers, ahead of Hudson because Belliard offensively is "maybe a little fresher."
Padilla will be the starting pitcher for Game 3 of the series, following Randy Wolf and Clayton Kershaw, with Billingsley penciled in for Game 4 and Jon Garland moved to the bullpen. Padilla is 4-0 with a 3.20 ERA and he struck out 10 in five innings against the Rockies on Sunday to secure his spot.
"I'm really happy. I'm grateful for the opportunity," said Padilla, signed by the Dodgers after being released by Texas amid criticism for his attitude. "It feels good to know they have confidence in me in this game. This is a second life that I have been given and I'm grateful for it."
Billingsley, although the club's leading winner with 12, spent much of September fixing a mechanical flaw and Monday pitching a simulated game to instructional league hitters in Arizona.
"Padilla, to me, has been throwing the ball really well," Torre said, "and I know Billingsley is getting there, otherwise he wouldn't even be considered for No. 4 if I didn't think I was more comfortable watching him."
Belliard said he ran the bases in a Monday workout and believes he will be close to 100 percent recovered from a left groin strain suffered 11 days ago.
"We'll go day to day right now against [Cardinals Game 1 starter Chris Carpenter], and we'll see what we have," said Torre. "We asked O-Dog to do a lot of stuff early in the year and he played his tail off and he continues to do that. But it was just a decision I decided to make."
Hudson took the high road, saying he saw the writing on the wall as he lost playing time to Belliard in September.
"You have to brush it off, that's all you can do," he said. "I can't get angry."
The 25-man roster is not required to be submitted until Wednesday morning, but it appears virtually set. Torre notified Jeff Weaver that he would be active as a situational right-hander to give St. Louis hitters a different look, with Garland serving as long reliever.
Among those apparently left off the pitching staff are right-handed relievers Guillermo Mota and James McDonald and left-hander Scott Elbert. Elbert was sent to Arizona to stay sharp in instructional league games.
Torre wants the left-handed power threat of Jim Thome, even though he can't run or play defense because of injuries. Mark Loretta, who can play both corner infield positions and bats right-handed, apparently had the edge for the final bench spot over Doug Mientkiewicz, who hits left-handed and is limited to first base. Position players Blake DeWitt and Jason Repko will not be active and are already in Arizona working out.
So, the roster likely will be starting pitchers Wolf, Kershaw, Padilla and Billingsley; relievers Jonathan Broxton, George Sherrill, Hong-Chih Kuo, Ronald Belisario, Ramon Troncoso, Garland and Weaver; outfielders Manny Ramirez, Matt Kemp, Andre Ethier and Juan Pierre; and infielders James Loney, Rafael Furcal, Casey Blake, Juan Castro, Thome, Belliard, Hudson and Loretta; and catchers Russell Martin and Brad Ausmus.
If that holds up, all five in-season acquisitions -- Sherrill, Belliard, Padilla, Garland and Thome -- will be on the postseason roster.
Torre's Game 1 lineup: Furcal (shortstop), Kemp (center field), Ethier (right field), Ramirez (left field), Loney (first base), Blake (third base), Belliard (second base), Martin (catcher) and Wolf.
Torre said he saw good signs from Ramirez in Saturday night's NL West clincher, including an RBI single, after his four-strikeout nightmare Friday night.
"He understands his responsibility here and he tries to get too big and really it affects his balance," said Torre. "I was a lot more comfortable with what I saw Saturday. And he seems to be in a good frame of mind. So, again, he's been down this road before as far as postseason and I expect him to show up for sure."
Torre said the club was boosted by the appearance of Hiroki Kuroda on the field Tuesday, playing catch for the first time since he was diagnosed with a herniated disk in his neck. Kuroda was scratched from the Division Series, but Torre said the club was optimistic that Kuroda would be available if the Dodgers advance to the NL Championship Series.
Kuroda said the "excruciating" pain he experienced last week had improved to "muscle tightness, similar to what I had after I got hit" in the head with a line drive on Aug. 15.
Ken Gurnick is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.