The Marlins acquired Lee from the Houston Astros in a trade on Wednesday, sending a pair of minor leaguers to Houston.
"It's a huge move, I think, the front office, showing people how much we want to win," Guillen said. "They show how much we care about winning this year, they showed the players that they're willing to do anything to help this ballclub."
The Astros acquired third baseman Matt Dominguez and left-handed pitcher Rob Rasmussen in the deal. Marlins general manager Michael Hill said the club also received cash considerations from Houston in the deal.
"We felt like it was a good time to infuse a veteran, professional, experienced bat into the lineup," Hill said.
The 36-year-old Lee spent five-plus seasons with the Astros and is hitting .287 with five homers and 29 RBI this year.
Houston manager Brad Mills removed Lee in the seventh inning of a 6-4 loss to the Pittsburgh Pirates on Wednesday. Lee's locker was already empty by the time the clubhouse opened after the game.
The right-hander gives the Marlins a veteran hitter as they try to get back into the race in the NL East. First base has been a problem for Miami this season, where regular Gaby Sanchez came into Wednesday's game hitting .194 with two homers and 16 RBIs.
Sanchez hit his third homer in Wednesday's 7-6, 10-inning victory over the Milwaukee Brewers, but it wasn't enough. Hill said Sanchez had been optioned to Triple-A New Orleans.
"Unfortunately, first base has not been a productive position for us, and we're looking to upgrade our offensive production at that position," Hill said.
Guillen provided a more harsh assessment of Sanchez's play.
"It's not easy, but that's our job," Guillen said. "I don't think he should be blaming anybody. He should blame himself. We gave Gaby a lot of opportunities. The reason they made this move (is) obvious. We've not had much production from him, and in Carlos, we hope we've got more production. People don't make moves just to make moves."
Hill said Lee did not have to approve the trade because the Marlins were not listed on his limited no-trade clause. He is expected to join the team in Milwaukee on Thursday.
"Still a dangerous hitter," Hill said. "He'll fit nicely in the middle of our lineup. He's a proven run producer, and we're expecting him to come in and do what he's done his entire career."
That's what Guillen will be counting on, especially with runners on base.
"He will bring those guys in," Guillen said. "He knows how to hit in an RBI situation."