Rich Harden, who is 5-1 with a 1.66 ERA with the Cubs, had been slated to start the second game of the four-game series against the Mets but was pushed back to Thursday.
Marshall will be followed by Carlos Zambrano (14-6, 3.77 ERA).
There is nothing wrong with Harden to prompt the Cubs to push back the right-hander, Piniella said.
"Harden says it's the best he's felt," Piniella said on Monday, "but we're starting to get ourselves rearranged for the postseason, also. We don't have anything to tell you as far as our rotation for the postseason yet, but we will, some time during the Milwaukee series. We want to see everybody pitch, and we want to see who our opponent is."
Ryan Dempster (17-6, 2.99 ERA) is scheduled to open the Cubs' series at Milwaukee on Friday, followed by Ted Lilly (16-9, 4.17 ERA).
The rearranged rotation does not mean that the Cubs are ready to announce who will start Game 1 of the NLDS.
"I'm not going to talk about Game 1 or anything until we see everybody pitch and find out who our opponent is," Piniella said.
The Cubs will have a say in the matter. The Mets entered Monday with a 1 1/2-game lead in the NL Wild Card race and trail Philadelphia by 1 1/2 games in the NL East. If the Mets were to win the Wild Card, they would face the Cubs next week at Wrigley Field in the NLDS. Chicago clinched the NL Central on Saturday.
One pitcher Piniella does want to get a better look at is Zambrano, who threw a no-hitter on Sept. 14 but had to leave his next start, on Friday against the St. Louis Cardinals, after 1 2/3 innings.
"He needs work," Piniella said of the right-hander. "He missed two starts prior to the no-hitter [because of tendinitis in his right shoulder], and the last start was an inning-plus. He needs work, it's obvious. We've got him on his fifth day, and we'll let him work as much as possible Wednesday."
Marshall will be making his seventh start of the season and first since Sept. 7, at Cincinnati, when he gave up one run on three hits and two walks over six innings. The lefty is 1-4 with a 4.24 ERA in his six starts, and 2-0 with a 2.81 ERA in 25 relief appearances.
He may get another start on Sunday in the regular-season finale at Miller Park.
"Let's see what Sunday brings," Piniella said. "Let's get through this New York series."
In addition to the rotation, the Cubs are looking at adjusting the relief corps. Piniella said that the team is leaning toward keeping 11 pitchers for the playoffs but also has a scenario that could involve 12.
As for the position players, the Cubs have one decision to make among two or three players, and the final week will determine that.
"The people who have been here all year will be with us, and we have to look at the callups to see which of the callups we'll keep," Piniella said.
The Cubs now find themselves facing one of the teams they could face in October.
"We're probably in a little more relaxed mode than the teams we'll be playing," Piniella said. "We're going to play as well as we can. We've got some things yet to do. One, we need to clinch home-field advantage for the entire playoffs. We'd also like to go into the postseason with some momentum.
"Thirdly, the teams that we're playing between now and Sunday are all teams battling for playoff spots, and we have an obligation to go out and put our lineups on the field. I'm going to rest people from time to time, and we've altered our rotation, but we'll play as representative a club as we can."
Carrie Muskat is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.