Soriano, who had been hoping to put off the MRI until after the weekend, will most likely get a cortisone shot to relieve the pain if he doesn't have a tear or other damage to the knee.
"It's something that needed to be done, so the quicker the better," Cubs manager Lou Piniella said.
Piniella said if Soriano gets a cortisone shot Saturday, he could be ready to play again by Tuesday. But Piniella said he didn't want to put Soriano in left field when his movement is limited.
Soriano hit a three-run shot Friday for his first homer since July 29, lifting the Cubs to a 5-2 victory over the 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.
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