"We all know what Freddy can do," Guillen said. "We know what kind of warrior he is."
Garcia has forged a 118-77 career mark in the Major Leagues, but he hasn't had much success since winning 17 games for Chicago in 2006.
"Freddy and me are close," Guillen said. "I talked to him and told him this is not Spring Training for him. He has to come ready for the challenge. He texted me, 'I'm ready to roll.'"
Garcia owns a 2-6 record in 14 starts with the Philadelphia Phillies and Detroit Tigers since the White Sox traded him -- for Saturday's two starting pitchers, Gavin Floyd and Gio Gonzalez -- in December 2006.
He made two starts for the New York Mets' Triple-A affiliate in Buffalo this year but was released in April because of poor results.
The White Sox signed the two-time All-Star to a Minor League deal just before his 34th birthday in June. He's made one start in Charlotte and is a combined 0-3 with a 6.35 ERA.
"He knows the situation here," Guillen said. "He has a lot of fans on this ballclub, and if he can pitch well for anybody he can with us."
Garcia was scheduled to throw a side session with the Knights on Saturday and will join the team in Chicago on Monday. The White Sox will announce a corresponding move before Tuesday's game.
Rick Eymer is a contributor to MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.