CHICAGO -- Brewers center fielder Tony Gwynn Jr. left Thursday's game against the Cubs with tightness in his left hamstring and worried that it might derail his quick start to the season.

Gwynn, who endured what he believes was a more serious hamstring injury last season, felt tightness before his first at-bat in Thursday's game but played on, attributing the sensation to scar tissue. A more pronounced "grab" as Gwynn tried to leg out a groundout in the fifth inning finally forced him out.

Gabe Kapler replaced Gwynn in the bottom of the sixth.

"I was through this once before last year, and I think they decided it was better to treat it and try to get it knocked out as soon as possible," Gwynn said. "I think we're being cautious with it right now."

The 25-year-old Gwynn was one of Milwaukee's heroes on Opening Day, when he collected two hits, bunted one eventual go-ahead runner into scoring position and then drove in the winning run with a sacrifice fly in the 10th inning. He did not start the middle game of the series Wednesday, because the Cubs started a left-handed pitcher, but he was back in the lineup for Thursday's finale against Chicago right-hander Ryan Dempster.

Gwynn went 1-for-3 and scored a run and is 4-for-7 (.571) so far this season.

"It's extremely frustrating for me," he said. "I feel like I'm just now getting into a rhythm at the plate. At the same time, it's part of the game."

He was expected to get the majority of starts in center field while Mike Cameron serves a 25-game suspension. The Brewers also planned to use Kapler and Gabe Gross at that position and would lean on those two players should Gwynn miss time.

The injury is not as bad as last year, when Gwynn returned from his dad's Hall of Fame induction ceremony in Cooperstown, N.Y., and pulled the hamstring trying to steal third base at Triple-A Nashville. After that game, he could not walk.

"I'm grateful [this time], because I can still walk," he said. "It's tolerable. Last year when I did it, I knew as soon as I got in the locker room that it was going to be a long time."

The team will re-evaluate Gwynn on Friday, when the Brewers play their home opener against the Giants at Miller Park.