Smith, who was acquired from the Angels on Aug. 1 for Minor League pitcher Jesus Castillo, has given up three home runs over 3 2/3 innings in six games with the Cubs.
"I'm not 100 percent and I was trying to go through it, and I'm obviously not getting the job done out there," Smith said. "I think everybody felt it was in the best interest to get to 100 percent and do what I was traded here to do. It's frustrating. I just want to get back and healthy and help this team."
Smith went on the disabled list in May because of a hamstring injury.
"I started changing my delivery in the lower half and created a bunch of problems, and it took awhile to get back," he said.
He's felt bad that he hasn't been able to contribute more to the Cubs since he was acquired.
"Coming to a new team, you want to do everything you can to help them win," he said. "It [stinks] when you're sitting here in August and not helping. Everybody's sore, everybody's hurting a little bit. It's August, that's how it goes.
"I know I'm not right. I don't think I've ever given up three homers in a week, especially against right-handed hitters. Something's not right."
Cahill started the first game of the Cubs' doubleheader on Tuesday, and he went five innings to pick up the win in a 4-0 victory over the Brewers. Cahill began the season with the Cubs, and appeared in 33 games in relief before he injured his right knee and had to go on the disabled list July 9 with patellar tendinitis.
As a reliever, Cahill posted a 3.07 ERA, and was holding opponents to a .209 batting average against. In his start on Tuesday, he gave up two hits over five scoreless innings.
• John Lackey, who came out of his start Sunday when his right shoulder became tight, will be pushed back in the Cubs' rotation and make his next start against the Padres on Tuesday. Lefty Mike Montgomery will make a spot start on Saturday against the Rockies in Denver, which would've been Lackey's turn.
"I think Montgomery could be a very legitimate Major League starter," manager Joe Maddon said. "This is the part of our group that we've been trying to get thicker. I'll take it at this time of the year."
The Cubs' rotation for the three-game series against the Rockies will be Kyle Hendricks on Friday, followed by Montgomery and then Jason Hammel. Jon Lester, Lackey and Jake Arrieta will face the Padres next week for a three-game series in San Diego.
"Any time a pitcher has a shoulder injury, you treat it as carefully as possible," Cubs general manager Jed Hoyer said. "He's been an incredibly durable pitcher over his career. We'll evaluate it every day."
If Lackey is not able to pitch Tuesday, the Cubs most likely would put him on the disabled list, Hoyer said.
Montgomery made two starts for the Mariners on July 10 and July 17 before he was dealt to the Cubs on July 20 in a four-player deal that included Dan Vogelbach. In his two starts, Montgomery has given up four earned runs over 11 1/3 innings for a 3.18 ERA. He's made 37 relief apperances, and posted a 2.29 ERA.
• Pedro Strop, who underwent arthroscopic surgery to repair a torn meniscus in his left knee on Friday, was playing catch on Wednesday, making about 30 throws from 60 feet. He will be sidelined four to six weeks.