Soriano was examined during Tuesday's game, a 15-6 loss to the Washington Nationals, by the team's medical staff.
"[The doctor] thinks it's nothing big," Soriano said after the game. "He said if I keep playing, and I have a sore knee, we'll do an MRI. I know I want to do it anyway, but he said if I play tomorrow or Thursday and it's sore, we'll do it right away."
Soriano, who is batting .116 in his past 12 games and .240 for the season, took early batting practice Tuesday but didn't warm up much before the session. His left knee bothered him then, and he thought it would feel better after treatment and regular batting practice, but it didn't.
Lou Piniella talked to Soriano after he hit in the cage, and decided to pull him. Jake Fox, who had four hits, including a solo home run, on Sunday against the Dodgers, started in left field in the Cubs' series opener against the Washington Nationals.
"It's been tight for three, four days," Soriano said. "I want to do the best I can. I've been playing with a bad knee for four months. One day it feels good, one week it feels good, two weeks bad. Sometimes it's 80 percent, sometimes 90 [percent]. It depends on the weather and how the treatment goes.
"It's more important to see if they can do an MRI today or tomorrow to clear my mind," Soriano said. "It's more mental; I don't know what I have in my knee."
He hoped to meet with the team physician during Tuesday's game. Soriano wanted to wait for the test results before talking about possible postseason surgery.
Meanwhile, outfielder Reed Johnson was able to take batting practice Tuesday but is not close to returning. He has been sidelined since he fouled a ball off his left foot July 30.
"Obviously, I'm disappointed," said Johnson, who was expected to miss at least four weeks. "I wanted to come back two weeks after the fracture. Then reality kind of sets in. I can hit and throw, which is good. I'll be ahead of schedule when I do come back."
But Johnson can't run or jog,and is limited to walking on the treadmill at a conservative 3 mph.
"Unless I'm guaranteed home runs every time, I can't play," Johnson said. "[I need a] ghost-runner."
Johnson will have another X-ray later this week to see how the bone is healing. It will be tough to find some place to rehab since the Minor League teams are wrapping up their regular-season schedules. He didn't make the West Coast trip in which the Cubs went 2-5.
"Watching is tough," Johnson said. "When you're not there and not a part of it, you know there's nothing you can do sitting at home on the couch. It's definitely frustrating to watch.
"I think some of the comments Rich [Harden] made about how everybody's saying, 'There's time left in the season, there's time left in the season' -- well, there's really not any more," Johnson said. "It's time to put up or shut up for us. Guys are starting to realize this is a time where even if we do play really well, we'll have to get some help from other teams."
Carrie Muskat is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.