"I thought it was just a normal bruise -- with swelling and tightness -- and I felt like the pain was enough to deal with to stay out there and pitch," Marquis said.
So he did, through the end of third inning and into the fourth, before falling to the ground after hitting Josh Thole with an inside sinker.
"Obviously, on that last pitch to Thole, pushing off, I just felt something pop," Marquis said.
"The doctor was saying he believes the bone in my leg broke on the line drive ... He said [pushing off] pretty much completed the fibula break, so it is what it is."
The 32-year-old veteran right-hander was acquired by the D-backs in a trade with the Nationals on July 30. In two starts prior to Sunday, he recorded an 0-2 record and 12.38 ERA. In 3 1/3 innings on Sunday, he allowed a run on three hits, and struck out two and walked one.
"I felt like today, when I was throwing the ball, I was getting closer to the results that I wanted," Marquis said. "But to end on this note. ... Hopefully, it's not over. Hopefully, I can recover from this quickly."
After being examined by trainers, Marquis limped off the field under his own power.
"What are you going to say?" Gibson asked. "That's our sixth fracture of the year. I told [team doctor] Michael Lee to stay out of here. The X-ray machine has been working."
About Marquis' injury, the manager said: "He wanted to stay in the game. When he planted it, he twisted it and felt a little something more. So I don't know exactly how it happens, but it does. He'll be out 4-6 weeks, at least. So we'll have to do something."
What something will be, Gibson said he would discuss on the team's flight to Philadelphia on Sunday evening.
Possible options include Micah Owings, Zach Duke or a Minor League prospect.
Marquis was wearing a boot, leaning on crutches and hoping for a return to the pennant race after the game.
"I'm frustrated," he said. "But this team has a lot of good young talent. ... I don't see why they can't continue and make a good run. And hopefully if this heals up, I can come back and contribute."