The MRI merely revealed some tendinitis. Team orthopedic specialist Dr. Steven Gryzlo will examine Soriano's knee on Sunday morning to determine if he needs a cortisone shot. Either way, Soriano likely will miss the next three games.
Soriano hit a three-run shot Friday for his first homer since July 29, lifting the Cubs to a 5-2 victory over the New York Mets.
After the game, Soriano said the pain in his knee had worsened. He struggled defensively in the win and dropped a fly, leading to boos at Wrigley Field.
"I want to play, but it's very tough," he said. "I don't get a very good jump because I cannot put weight on my knee."
Soriano, who signed an eight-year, $136 million deal before the 2007 season, is batting .243 this year. Friday's blast gave him at least 20 home runs in eight straight seasons.
Bruce Levine covers baseball for ESPNChicago.com. Information from The Associated Press was used in this report.