CHICAGO -- After all the concern about Mark Prior's innings and workload, the right-hander's rookie season is over because of a hamstring.

The Chicago Cubs placed Prior on the disabled list Monday with a strained hamstring, an injury that will take at least three or four weeks to heal.

"It makes no sense thinking about getting him back for the last week of the season," general manager Jim Hendry said. "He did a terrific job, obviously, but he's the cornerstone of our future and we're not going to take any chances with him."

Prior was injured Saturday in the first game of a day-night doubleheader against the St. Louis Cardinals. Batting in the bottom of the fifth, he ran to first base after a third-strike wild pitch by Cardinals starter Luther Hackman. He then slid hard into second on a fielder's choice.

Prior came out to start the sixth, but was unable to deliver a pitch.

"Everyone was worried about how many pitches I'd thrown and how many innings, and it's a hamstring," Prior said. "I guess if I've got to go on the DL, I'd rather go on for a leg injury than an arm injury."

Still, he isn't happy about being on the disabled list for any reason.

"Obviously I'm upset. I didn't want to go out this way," he said. "It's tough because I'm happy with what I did but I'm not happy with what I did."

Prior, the No. 2 pick in the 2001 amateur draft, went 6-6 with a 3.32 ERA in 19 starts with the Cubs. His 147 strikeouts were third-best on the team behind Matt Clement and Kerry Wood, and he walked only 38 in 116 2-3 innings.

He pitched one complete game, a 4-1 victory over the Colorado Rockies on Aug. 4, his 14th start. Two starts later, he tied a Cubs record by striking out seven in a row against the Houston Astros.

"It was very good, especially considering the experience he had coming in. Or the lack of it, really," Larry Rothschild, the Cubs pitching coach, said of Prior's season. "For a young pitcher to go through some of the stuff he's gotten through and come out the way he has speaks volumes."

Indeed, Prior has had a meteoric rise since the Cubs drafted him. He came to spring training not having pitched since the College World Series, yet dazzled the Cubs and their opponents with his potential.

Starting the season at Double-A West Tenn, he tore through the minor leagues. He went 5-2 with a 2.29 ERA in nine games at Double-A and Triple-A, striking out 79 while walking only 18.

He made his major league debut on May 22, striking out 10 and allowing two earned runs in six innings as the Cubs beat the Pittsburgh Pirates 7-4. He was only the 14th Cubs starter since 1920 to win his big league debut.