But Marshall, who has a smooth delivery, finished that pitch to Fick in an odd way. Cubs catcher Henry Blanco pointed to the dugout, and Baker, pitching coach Larry Rothschild and assistant trainer Ed Halbur ran to the mound.
"I just felt it when I was throwing off the mound," Marshall said. "Hopefully, I can come back as quick as Mark has."
"It's basically the same thing," Cubs athletic trainer Mark O'Neal said when asked if Marshall and Prior had similar injuries. "I'd put [Marshall's as] a little more severe than what Mark had."
Marshall was tested after the game, and he felt some discomfort. When asked to force a cough, the pain was pretty significant, O'Neal said.
"Right now, I'd say he has a pretty good chance of going on the disabled list," O'Neal said. "We'll see how he is [Sunday]."
Among the Triple-A Iowa candidates the Cubs could call up are left-hander Rich Hill, who is 6-1 with a 1.94 ERA, or right-hander Angel Guzman, who is 3-4 with a 4.01 ERA.
Marshall said he's never had a problem with his oblique muscles, and he thought he might have been dehydrated and the problem was related to the excessive heat. It was 92 degrees at game time.
"I kept pumping fluids the whole game," Marshall said. "Hopefully it's one of those freak things and I can get back on the mound."
This was Marshall's 19th start, and he's now totaled 103 innings. The rookie made 16 starts last season between Class A and Double-A and totaled 99 innings.
"I feel great, I feel strong," Marshall said. "I was kind of a little wild today. I had a couple grip issues. We played good defense and they got a couple runs for me there. It was a good game other than a couple innings."
The Cubs lost, 7-3, to the Nationals despite a second straight two-homer game by Aramis Ramirez.
Saturday's game was the 35th start by a rookie pitcher this season for the Cubs, and they are 8-17 with a 5.74 ERA, giving up 116 earned runs over 182 innings. Carlos Marmol will make the 36th rookie start on Sunday in the series finale.