Greinke needed 40 pitches to get four outs, walking three and allowing one hit Tuesday as the Brewers beat the Chicago White Sox 3-1.
Acquired from Kansas City in December, Greinke loaded the bases in the first and fell behind 2-1 in the count to Dayan Viciedo, prompting a visit to the mound by pitching coach Rick Kranitz.
Greinke got Viciedo to swing and miss at two straight breaking balls to escape trouble, then was removed after walking Brent Lillibridge in the second.
"It's not as bad as the numbers look," Greinke said. "I was just missing up with the fastball. It wasn't perfect but it felt good coming out, so that's a good sign."
Greinke said the bruised right side was caused by a "stupid" off-field incident he wouldn't elaborate on.
"I didn't really notice it today," he said. "I could completely take off and it would get better faster, but instead it's getting better slower and I'm a little bit behind."
Mark Buehrle pitched two perfect innings with one strikeout, needing just 12 pitches in each inning. He threw 15 pitches for strikes.
"It's only two innings, but physically, location, everything was there," he said. "I say it every spring, but I hope I feel this good opening day."
Alejandro De Aza had an RBI grounder for Chicago in the ninth.
Adam Dunn struggled again as the White Sox designated hitter, striking out, walking and hitting into a double play. On Monday, he walked once and struck out twice Monday.
The double-play ball in the fifth would have gone through the middle, had the Brewers not applied an infield shift. Weeks, the second baseman, was right behind the bag when he fielded the ball.
What White Sox manager Ozzie Guillen took from the game was a promising day defensively from Viciedo, who made the switch from third base to outfield.
"He has matured a little bit. He came into camp in shape ready to play," Guillen said.
NOTES: White Sox RHP Jake Peavy said he felt fine after a 40-pitch bullpen session, despite some fatigue during the final 15 pitches. Peavy is slated to start Friday's game against the Angels. ... Brewers CF Carlos Gomez and 3B Craig Counsell both made exceptional defensive plays in the fourth to prevent a big inning.