Olsen rolled his right ankle while covering first base in the fifth inning on Saturday night in Florida's 8-6 loss to New York at Shea Stadium.
The 24-year-old sprinted to the bag on Carlos Delgado's grounder to first baseman Mike Jacobs. The flip from Jacobs caused Olsen to extend, and the pitcher stumbled to the ground. He ended up tagging the base with the ball in his glove to end the inning.
Olsen walked off the field with a slight limp that inning. He faced one batter in the sixth inning and exited after Fernando Tatis' single.
The left-hander is lined up to start on Thursday against the Cardinals at Dolphin Stadium.
"It's not that big of a deal right now," Olsen said on Sunday.
But the southpaw didn't do his normal day-after running. He threw off flat ground, and he is hopeful to make his typical between-starts bullpen session, which would be on Monday.
With 147 innings pitched on the season, Olsen said missing a bullpen session at this time of the year isn't a reason for concern.
Marlins manager Fredi Gonzalez is confident that Olsen could be available for Thursday.
"In talking with the training staff, they seem to think he's going to be OK," Gonzalez said. "We'll know more the closer we get to that date."
Olsen is 6-7 with a 4.04 ERA this season.
He's been getting ice treatment, and the ankle has some swelling, but not a great deal.
Olsen has not had any medical tests taken, which is another reason the club is confident the ankle won't keep him out on Thursday.
Added Gonzalez on whether Olsen will start against the Cardinals: "I'll be surprised if he doesn't."
If Olsen isn't ready for Thursday, a possible candidate to go that day is Andrew Miller, who has been on the disabled list since July 18 with right patella tendinitis.
The left-handed Miller threw 3 2/3 innings in a rehab assignment on Saturday night for Class A Jupiter. Miller is in line to toss again on Thursday, when, for now, he is slated to throw about 75 pitches for Jupiter.
Olsen was his typical upbeat self on Sunday. When the team leaves New York following its afternoon game with the Mets, Olsen will be bringing a new Brett Favre No. 4 New York Jets jersey back to Florida. The jersey was picked up by Marlins outfielder Luis Gonzalez, who had the No. 4 shirt placed in Olsen's locker.
A fan of the Chicago Bears, Olsen has a great deal of respect for Favre. Olsen added that Favre could have gone anywhere and he would still be a fan.
"Brett Favre is the coolest NFL player ever," Olsen said. "By far."
Joe Frisaro is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.