CHICAGO -- The combination of Tuesday's rain-induced cancellation of a game against the Yankees and no off-days until this Thursday temporarily threw the White Sox starting rotation out of whack, with Sunday's series finale against the Cubs serving as the object of consternation.

Ozzie Guillen and his crew tossed around a number of different ideas to bridge the gap: bring back Jose Contreras or John Danks on three days' rest, call up Gio Gonzalez or Gavin Floyd from the Minors for one start, or bring in Nick Masset from the bullpen for a spot start.

By Wednesday night's very late postgame, the White Sox manager seemed to have settled on Contreras as his starter. By Saturday morning's pregame conversation with the media, Guillen had changed his mind.

Masset, who turned 25 on Thursday, will make his first Major League start under the glare brought about from one of baseball's top rivalries. The right-hander joins Felix Diaz (June 26, 2004) and Brandon McCarthy (May 22, 2005) as talented young White Sox hurlers pressed into starting duty for the Crosstown Showdown.

But Masset doesn't intend to think about the ramifications of this opportunity until after it has been completed.

"Tomorrow, it's my job to be on the mound and keep my team in the ballgame," said Masset, after the news of his start became official. "I look forward to that. I'll do my best and take it one pitch at a time."

According to Guillen, the plan to start Contreras began to change a couple of days ago when the big right-hander expressed concern in coming back on short rest. Contreras threw 100 pitches in the White Sox second-game loss to the Yankees on Wednesday, and the team has always been careful to err on the side of extra rest where Contreras' preparation is concerned.

There also was no desire on the White Sox part to mess with the regular routine of Danks, who has been one of the most impressive rookies in baseball through the season's first seven weeks. With those two candidates ruled out, Masset became the prime option -- Masset, and a little help from the remaining six relievers.

"I don't want to lose my ace," Guillen said, regarding moving Contreras back to his regular turn Monday against the A's, followed by Danks on Tuesday. "I want to keep as fresh as we can, all my pitching staff. Masset [will pitch]; whatever he gives me, we will appreciate what he does."

"We didn't feel comfortable bringing either one of them back," White Sox pitching coach Don Cooper added. "They both had thrown a good amount of pitches, and they are going pretty good."

Cooper characterized Masset's special rotation slot as a start but also as a "relief start." The youngster's longest appearance out of the bullpen this season came on April 5, when he worked 4 2/3 innings after Mark Buehrle was struck by a Ryan Garko line drive in the second inning of a game against Cleveland.

That particular contest also marked Masset's highest pitch count at 66. Masset understands he will be under different restrictions than the regular starting five when he takes the mound Sunday, but hopes he can get the White Sox four or five innings.

"I'm going to go out there and try and get hitters out early," Masset said. "But whatever pitch count they have me on is when I'm going to be pulled out of the game.

"I feel really good about it and it's a great honor. [Starting is] something I've always done in the past, and I look forward to giving my team the best chance to win."

As a Minor Leaguer in the Texas system, Masset made 104 starts in 157 career appearances from 2001 through last season. This total includes 15 starts combined during the 2006 campaign. But all 19 of Masset's Major League appearances have come in relief.

Adding Masset to the rotation, even for one day, puts a little extra pressure on the rest of the starting staff. Even with just a four-inning outing on Sunday, Masset probably will be lost to the White Sox relief crew until Wednesday's series finale against Oakland.

It's a chance the White Sox are willing to take, with a focus on protecting their most important commodity over the season's long haul.

"Our starters are very important to us the rest of the season," Cooper said. "We don't want to rock their boat a little or maybe ask them to do something that they are not quite physically up to right now."

"[Jose] said he feels great," Guillen added. "He feels he's throwing the ball really good, but he was worried about coming back early then all of a sudden going backwards."

Growing up in St. Petersburg, Fla., Masset became a fan of both the Cubs and Braves by watching them on their respective Superstations. His biggest challenge, though, won't come from shutting down the team he used to view on television as a kid.

Instead, it's going to be trying to get a hit off Carlos Zambrano, a pitcher that has some of the best pure stuff in all of baseball, after Masset hasn't really hit since high school. But as a pitcher, at least Masset has history working on his side.

Diaz not only faced Zambrano in his own spot start three years ago, but also defeated the Cubs ace at U.S. Cellular Field.