SAN FRANCISCO -- Asked to define his role on the Cubs' pitching staff, Tyler Chatwood said, "You tell me," prompting laughter from reporters.

If Chatwood continues to perform as he did in Chicago's 4-1 victory Wednesday over the Giants, the Cubs will find him suitable responsibilities to occupy. They wouldn't want to waste that right arm of his.

Despite being not completely sure until 10:30 a.m. that he would start against the torrid Giants, who had won 17 of their previous 20 games, Chatwood did just fine. He allowed one run and four hits in four innings -- not qualifying for the win, but certainly setting the Cubs on the proper course.

This was supposed to be Jon Lester's start, but the big left-hander began feeling feverish Tuesday. This forced the Cubs to call on Chatwood, who was all too happy to participate in a game.

Performing as an extended opener, in a sense, Chatwood yielded four hits and San Francisco's sole run while walking one and striking out five. Falling one inning short of qualifying for the victory didn't bother Chatwood one iota.

"I got a chance to pitch," said Chatwood, who has made 22 appearances for the Cubs, including four starts. "I was excited to pitch."

And for good reason. Chatwood has thrived against the Giants, particularly while pitching for Colorado, one of their National League West rivals. Chatwood owns a 7-4 record and a 3.01 ERA in 19 lifetime appearances (18 starts) against the Giants, including a 4-2 mark with a 2.04 ERA in nine starts here.

Chatwood was nearly eliminated from consideration for pitching Wednesday, because he almost entered the Giants' 5-4 win Tuesday. Cubs manager Joe Maddon said Chatwood would have pitched the 14th inning -- and beyond, if necessary -- had the game extended beyond the 13th. Then the Cubs might have had to rush a Minor League starter here to face the Giants.

After Wednesday's game, the Cubs announced that Lester will start Saturday at Milwaukee.