Carlos Zambrano picked up the win, but it could prove costly. He left the game with a strained left hamstring and may miss his next start. Zambrano (3-1) was injured when he stepped awkwardly on first base in the fifth and was pulled with the game tied at 2.
"That will be a big blow," Lee said of the possibility of losing Zambrano. "He's a horse, and we don't want him to miss too many games."
Lee's homer was his second in as many games and his first slam since May 19, 2007, when he came off the bench to deliver a pinch-hit homer against the Chicago White Sox.
The game was tied at 1 when Mike Fontenot belted his fifth homer with one out in the Chicago fourth. He now has five homers on the season; he didn't hit his fifth last year until his 63rd game of the season.
The Marlins answered. They had runners at first and third with one out in the fifth when Ricky Nolasco (1-3) bunted toward third. Geovany Soto picked up the ball and threw to first, somehow missing the easier play on the runner at third, Jeremy Hermida, who was halfway down the line and scored on Soto's throw.
"That play there, it's right in front of you," Piniella said. "That play there, you can see the runner was sitting there in no man's land, waiting for a throw to first to score."
Zambrano did lecture Soto in the dugout after the inning. Big Z then led off the fifth with a bunt single, ran full speed to first, and appeared to reach for the bag with his left leg. He came up limping and was pulled, and one out later, the Cubs loaded the bases on Ryan Theriot's bunt single and an error by right fielder Ross Gload, who couldn't get his glove on Kosuke Fukudome's ball. Lee followed with his slam, a monster shot to straightaway center, to open a 6-2 lead.
"That's a big grand slam," Piniella said. "He got a breaking ball a little up in the zone and hit it right over the center-field wall. It's good to see. [If] we get Derrek swinging the bat the way he's capable, it really helps our offense."
Lee's average is finally over .200, and he's 4-for-12 in his last three games with two homers and five RBIs.
"It's getting better," Lee said. "I'd like to mix in some other hits with it, but I'll take home runs."
He even waved to the fans for a curtain call, although he was hesitant to step out of the dugout.
"It felt good," Lee said. "I didn't know if I wanted to come out because they've been booing me every at-bat, but it felt good."
On Saturday, Lee heard it from the Wrigley Field crowd when he struck out to strand two and end the second. He homered in his next at-bat that game.
"It doesn't bother me," Lee said. "I just go play the game, and I just concern myself with helping the team win. The fans are free to do what they want to do. It doesn't bother me one way or the other."
What does bother the Cubs a little is Theriot's offensive surge. The shortstop does like to show off his muscles.
"He always lets us know," Fontenot said of Theriot, who hit a grand slam on Friday, ending a string of 620 homerless at-bats. "He comes into the clubhouse looking for different things to say. It's been fun with him."
It's good fun.
"We've got a big wager going on," Lee said, laughing.
Lee belted 46 homers in 2005, when he won the National League batting title, but hasn't come close since. He says he's not worried about the power, or lack of, but wants good, consistent at-bats.
"I'm trying to get more on top of the ball and get through it," Lee said. "I feel like I've missed a lot of pitches and popped them up rather than driving them in the gaps or over the fence. Lou gave me a little hitting lesson [on Saturday], and it helped today."
The Cubs now have hit five homers in the last three games, all wins.
"We're playing a lot better," Lee said. "We've been so up and down this year, to go out and play three games and put some runs on the board and have solid pitching, that's more like us."
Carrie Muskat is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.