Piniella would not put a timetable on when Soriano might return to the lineup. He would not rule out that the veteran outfielder could be done for the season. There is a strong possibility Soriano will need a surgical procedure to have debris cleaned out of his left knee when the season is over.
"I had a talk with him before coming out here and the crux of it is his knee is not strong," Piniella said before the Cubs began a weekend series against the Mets at Citi Field on Friday night. "He's having trouble playing on it, hitting on it. In fairness to him, we're going to have him continue to do all his exercises with our trainer and revisit this thing down the road.
"There is no timetable," Piniella continued, adding that Sam Fuld, who has no homers and no RBIs to show for 41 at-bats prior to Friday night's game, would be one of Soriano's replacements. "We appreciate the fact that he wants to play, he's a gamer, that he wants to help us. But at this time and point, because of his knee, he is not able to do that."
Soriano, 33, has hit 25 or more home runs and driven in 70 or more runs in every season since 2002, but this year, he has 20 home runs and 55 RBIs in 117 games.
Soriano, who was limited to 109 games a year ago and spent two stints on the disabled list with calf and hand injuries, initially hurt his knee this season when he slammed into the outfield wall in a game against Cincinnati on April 22.
The outfielder had a shot in the knee on Sunday and was back in the lineup on Tuesday, but he took an 0-for-4 in a loss to the White Sox on Thursday and was 5-for-18 with one homer and three RBIs on the homestand that concluded Thursday.
"I cannot concentrate because my knee bothers me," Soriano told reporters before the game without revealing that he was being shut down. "My knee is a little sore and that's what I have in my mind right now."
Piniella said he pondered his decision for a long time. He thought about it on the team's charter flight from Chicago to New York on Thursday evening and spent most of the night considering Soriano's condition.
"I vacillated about it, and this morning, I still felt this was the best thing to do for this young man," Piniella said. "And I stick to it."
The manager was asked if Soriano might be done for the season.
"I'm not going to go that far," Piniella said. "Remember, he had a shot a few days ago. We gave him a few days off, but obviously it hasn't been enough. I had a nice talk with our trainer and strength coach on what we need to do to get it stronger, and we're going to go with him in that direction. And then we'll revisit it. I don't know when or how soon, but we'll revisit it."
Soriano said the knee has become a mental distraction to him only recently.
"The first time when I was sore, I wasn't paying attention, because I like to play this game," Soriano said. "Now, it's at the end and it has been getting worse. At the beginning, I wasn't even thinking about it."
Soriano said he'd like to avoid surgery, but he sounded like a man who didn't have much choice.
"They said they want to clean it up," he said. "Maybe after the season they want to clean it up and get ready for Spring Training next year.
"It's not big. It's like changing the oil when you have a car."
Kit Stier is a contributor to MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.