"We just didn't hold the game in check," skipper Lou Piniella said.
Making his third career start, Diamond lost control early.
The first run allowed was understandable. Diamond left a fastball up thigh-high over the plate, and Albert Pujols did what was expected -- turned on it and sent a deep blast to center field for the Cards' first run of the game. Diamond struck out Matt Holliday to end the inning, however, and that first frame ended up being the sharpest for the rookie right-hander.
In the second, the game started to slip, with Diamond walking St. Louis starter Jake Westbrook with two outs to turn over the batting order for the third inning.
St. Louis made him pay after leadoff hitter Felipe Lopez walked and was driven in by a Matt Holliday double. Yadier Molina pushed across Pujols with a sacrifice fly for a lead the Cubs would never erase.
Topping it off, Westbrook hit a ground-rule double off Diamond to lead off the fourth and later came around to score.
The final line? Four innings pitched, four runs, four walks and six hits for Diamond, who did turn several hard-hit balls into outs to prevent the game from turning into a blowout.
"It started out OK. The first couple innings were all right, then location just went out the window," Diamond said. "I just didn't do my job."
A potential replacement for Diamond in the rotation, Casey Coleman, came in and pitched two scoreless innings in his bid for a starting spot.
Piniella wasn't ready to declare which pitcher would get the next start, but he didn't rule out Coleman.
"We will see. No decision is going to be made today," Piniella said.
The Cubs started the game in hopeful fashion, scoring two first-inning runs off four straight hits from Starlin Castro, Derrek Lee, Marlon Byrd and Blake DeWitt.
But with runners on second and third, Alfonso Soriano grounded out weakly to third base before rookie Darwin Barney ended the inning with a lazy fly ball.
Chicago couldn't touch Westbrook after that, managing just two singles and a walk over the next five innings.
"These sinkerballers, usually you get to them early in the game. We had a chance there in the first inning to take a nice little lead -- which we did, with two runs, but I mean more," Piniella said. "And all of a sudden, his ball started to sink, and he got tough. He threw more breaking balls and a few more changeups than I have seen in the past. But he knows how to pitch."
Lee added the Cubs' final run with a solo shot in the eighth. The first baseman, activated from the bereavement list on Friday, went 2-for-4 with a double and the home run.
"Derrek Lee had a nice day," Piniella said simply.