NASCAR fans might get a look at open-wheel racing star Juan Pablo Montoya a lot sooner than expected.

Montoya, who announced Sunday at Chicagoland Speedway that he will drive for former boss Chip Ganassi next season in NASCAR, is leaving Formula One's McLaren Mercedes team immediately.

"Team McLaren Mercedes and Juan Pablo Montoya have mutually agreed for him to step down in the forthcoming races of this year's Formula One World Championship," McLaren CEO and chairman Ron Dennis said Tuesday. "We have agreed that, with so many things happening in Juan Pablo's life right now, he should take some time out of the car and prepare professionally and personally for the future."

Instead of resting up, though, Montoya could take advantage of the early release to get some experience in stock cars, a totally foreign form of racing to the 30-year-old Colombian driver, who has spent his entire career in open-wheel cars.

Ganassi told The Associated Press on Tuesday he does not yet know the details of the driver's release from McLaren, but that it means Montoya could race in Nextel Cup events this season.

"This is all new. But, if Juan is available earlier than we expected, our program for him will definitely get an early start," Ganassi said.

In making the announcement Sunday, Ganassi said that Montoya would replace Casey Mears in the team's No. 42 Dodge in 2007. Mears is moving to Hendrick Motorsports.

Ganassi also said Sunday that Montoya likely will make a few starts in the Busch Series after the F1 season ends in October to get started on what could be a tough transition.

Asked Tuesday if the possibility of having Montoya in NASCAR earlier than expected means he could also run a few Nextel Cup events before the end of this season, Ganassi said, "It's certainly possible, but that's about the third or fourth step in this whole situation. We need to know all the details first before we start making plans."

Montoya won a CART series championship in 1999 and the Indianapolis 500 in 2000, both while driving for Ganassi. During that time, the driver and team owner also became close friends, and Ganassi acknowledges he is excited by the prospect of getting back together sooner than expected.

"The thing about Juan is he adapts so quickly," Ganassi said. "That has always been one of the best things about him as a driver. It's going to be really interesting and fun to see him get into stock cars."

After his success in American open-wheel racing, Montoya moved to F1 with Team Williams in 2001 and was expected to become an instant championship contender. But he fell short of expectations with Williams and, after moving to McLaren in 2005, also was disappointing with that team.

He never finished better than third in the F1 drivers championship and won just seven races in 95 starts. Montoya was sixth in this year's F1 standings.

Now, he is bringing his all-out racing style to NASCAR, switching from the agile, 1,300-pound F1 cars to the 3,400-pound behemoths of NASCAR.

"I know that it will be a tough transition, but I'm really excited about the opportunity to move into the NASCAR championship," Montoya said Tuesday in a statement release by the McLaren team. "I have enjoyed most of my time in Formula One and I'm grateful for this opportunity to settle my personal life and concentrate on my future career."

He will be replaced by test driver Pedro de la Rosa for the rest of the Formula One season. De la Rosa will race alongside McLaren teammate Kimi Raikkonen at the French Grand Prix on Sunday.