Lilly (5-5, 5.23 ERA) and Dempster (7-2, 2.90 ERA) are slated to open the Cubs' homestand against the Atlanta Braves, which begins Tuesday. Sean Gallagher (3-2, 4.42 ERA) was originally scheduled to start Thursday, but Zambrano (8-2, 3.01 ERA) may be inserted into the rotation that day.
"We're looking at it," manager Lou Piniella said Sunday. "We're trying to get our top starters seven starts between now and the All-Star break."
Zambrano took the loss on Saturday against the Los Angeles Dodgers and would be working on normal rest. The Cubs also have an off-day on June 16, when they play the San Diego Padres in the Hall of Fame Game in Cooperstown. That game is an exhibition, and the Cubs will call up several Minor League players for the game. They are not expected to use any of their five starters.
The switch is being done to give their best pitchers an extra game -- and give the Cubs a better chance.
"If we stayed in rotation, everyone would get the same amount of starts," Piniella said. "It seems to me if we can get Zambrano seven starts, Dempster seven starts, Lilly seven starts, it would make sense.
"We're looking at it for the longer term, and not just this time around," he said.
The All-Star Game will be played July 15 at Yankee Stadium.
In a move to take advantage of Monday's off-day, Piniella gave shortstop Ryan Theriot a day off on Sunday, so he'll have two days' rest. Theriot had trouble catching two throws in Saturday's game, and he and infield coach Alan Trammell will do some extra work on Tuesday at Wrigley Field.
"One of the problems is you don't take infield hardly," Piniella said. "We just have to work on it. What happened yesterday can happen to anybody."
The same could be said of Zambrano's meltdown in the seventh inning of the Cubs' 7-3 loss to the Dodgers on Saturday. Piniella saw about 10 seconds of the temper tantrum on television Saturday night. He missed seeing it live because he was on the mound making a pitching change.
Zambrano had given the Cubs a 3-2 lead with an RBI single in the seventh, but the Dodgers rallied in the bottom of the inning.
With one out, Juan Pierre singled, stole second, and scored on a throwing error by third baseman Aramis Ramirez. One out later, Russell Martin lofted a single to right past a diving Kosuke Fukudome to drive in the runner and go ahead, 4-3. James Loney then singled to set up Matt Kemp's three-run homer.
That's when Zambrano made quite an exit. He kicked a Gatorade cooler twice in the dugout, picked it up and slammed it to the ground, then picked it up and tossed it again. He flung his cap in disgust, and disappeared into the clubhouse.
"He picked that Gatorade thing up like a beer keg," Piniella said. "I heard a lot of commotion. I didn't realize it was Mr. Zambrano providing the entertainment."
Zambrano had a few scrapes on his left hand from the incident.
"That's what I told him is, 'Don't get hurt,'" Piniella said.
Carrie Muskat is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.