Gonzalez underwent exploratory surgery with Dr. Thomas Graham, and a growth was found beneath the sheath around the muscle.
Rockies head athletic trainer Keith Dugger said these type of tumors are common and the expectation is a biopsy will determine the tumor is benign.
"These little benign tumors, which we hope it is, is probably the second most common finding in the finger besides cysts," Dugger said. "Usually repetitive trauma causes it.
"They call them cell tumors. Pretty common within the sheath of these fingers. Usually benign. Occasionally you can come up with something."
Gonzalez had his left arm in a sling after the surgery. He is expected to return to Denver on Wednesday.
"It helps explain some of the things that CarGo's been dealing with and why that finger kept blowing up on him," Rockies manager Walt Weiss said. "It helps answer some of the questions we had about him."
Gonzalez has been experiencing soreness and swelling in the finger for a while. He was examined by Graham on May 29 in Cleveland. He continued to play but was placed on the 15-day disabled list June 4 and is expected to miss a few weeks.
"Typically, it's a couple weeks for the tissue to heal," Dugger said. "Then he'll get back his strength and start swinging. It was a little bit more invasive, meaning they took out a larger piece than they thought -- what the MRI revealed."
Gonzalez has dealt with finger issues in the past. Last season he played in just 19 games after the All-Star break due to a sprained middle finger on his right hand.
That injury did not require surgery.
Colorado has been hit hard with injuries in the past month. Third baseman Nolan Arenado (broken left finger) is expected to miss another month, and outfielder Michael Cuddyer is out for at least six weeks with a fractured left shoulder suffered Thursday.
The rotation has been decimated as well. Jordan Lyles (broken hand), Tyler Chatwood (right elbow strain), Brett Anderson (fractured left index finger) and rookie Eddie Butler (right rotator cuff inflammation) are on the disabled list.
Left-hander Tyler Matzek, the team's first-round pick in 2009, will be the third Rockies pitcher in a week to make his major league debut when he starts Wednesday's game.
"It's something as a professional team we've got to push through, push forward," Cuddyer said.