The second baseman's back had tightened up on Wednesday after he overdid a weight-lifting session. He came on as a defensive replacement Friday, but was obviously in pain. He did not play Saturday.
"It was being foolish," DeRosa said of how he got hurt. "I was trying to do something productive with the day, and I went too heavy. That's the last time I'll do that."
Hitting isn't the problem -- running is.
"I can hit," DeRosa said. He then went out and looked fine during batting practice.
He was not in the Cubs' original lineup for Sunday's series finale against the Cincinnati Reds, but was added about a half hour before first pitch.
"We're trying to be a little cautious with him," Cubs manager Lou Piniella said before the game.
As for third baseman Aramis Ramirez, he may be available Monday for the Cubs' first home night game. He's been sidelined since Friday because of tendinitis in his right wrist.
"We're hopefully optimistic for [Monday] night. We'll see," Piniella said.
Taking it easy: Scott Eyre was hospitalized Saturday night because of flu-like symptoms. He was taken from Wrigley Field on Saturday on a stretcher.
"He's not at the ballpark," Piniella said Sunday of the Cubs left-handed reliever. "He's going to be released from the hospital [Sunday] afternoon. He'll be here Monday. He was really dehydrated more than anything else."
The Cubs have two other lefties in the pen in Neal Cotts and Will Ohman, and southpaw Ted Lilly started Sunday.
Extra bases: Derrek Lee addressed the Wrigley Field fans before Sunday's game as part of the Jackie Robinson Day celebration. Lee, Cliff Floyd, Jacque Jones, Daryle Ward and coaches Gerald Perry and Lester Strode all wore No. 42 in honor of Robinson. ... Lilly is following left-hander Rich Hill in the rotation, but Piniella said they'll try to break up the southpaws after their next starts. The two will stay in order this week because they'll be pitching against different teams. Hill is scheduled to face the Atlanta Braves on Thursday, and Lilly will face the St. Louis Cardinals on Friday. "We can break them up after that," Piniella said. "I'd rather not see them back-to-back against the same opponent." ... Cesar Izturis' successful suicide squeeze on Saturday was a result of the Cubs trying to figure out ways to manufacture runs in the cold weather. "Right now, we're trying to make something happen like that," Izturis said. ... Former Notre Dame quarterback Brady Quinn threw out a ceremonial first pitch Sunday.
Under the lights: Entering this season, the Cubs were 174-163 at night at Wrigley Field, a .516 winning percentage. Since playing the first night game Aug. 9, 1988, the Cubs have a .506 winning percentage in day games (568-554). This is the 20th season of night games at Wrigley. The Cubs have posted winning records at night at Wrigley 11 times and have had a losing record six times and a .500 record twice. Last year, they were 14-13 under the lights.
The Cubs won their first seven home night games in 2006, then lost the next six.
Minor matters: Carlos Marmol struck out 10 over 5 1/3 scoreless innings Saturday to help Triple-A Iowa beat Round Rock, 8-0. Marmol gave up five hits. Micah Hoffpauir hit a grand slam, and leads the PCL with 17 RBIs. ... Tennessee's game on Saturday was postponed because of the weather. ... Tyler Colvin had two hits, including a double and a triple, in Daytona's 10-5 loss to Jupiter. Colvin was batting .364. ... Rafael Dolis gave up four runs on five hits over four innings in Peoria's 8-5 loss Saturday.
On deck: Jason Marquis is slated to start Monday night in the Cubs' first home night game of the season, with first pitch set for 7:05 CT. The Cubs play host to the San Diego Padres for two games, including Tuesday's matchup against Greg Maddux.
Carrie Muskat is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.