"We have a chance to either go that day or give him a couple of extra days," Mattingly said Saturday morning. "We'll see where he's at. I just want to make sure that he's 100 percent. I don't want anything going backwards. We've got plenty of time at this point, still."
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Cosart was shaken up in the third inning of Friday's 5-4 win over the Nationals. The 25-year-old had his knee rolled on at third base by Martin Prado.
Cosart completed the inning, throwing 27 strikes and striking out three. He allowed one hit and issued a walk.
After being examined by the club's medical staff, it was determined Friday that there was no ligament damage.
On Saturday morning, team trainer Sean Cunningham reported to Mattingly that there was no swelling in Cosart's knee.
"It sounded good, but on the other side of that, we'll still be cautious," Mattingly said. "There was no real swelling coming in today, according to Sean. But again, we will be cautious, and make sure that he is 100 percent before we send him back out there."
Cosart is in the mix for one of two rotation openings.
The right-hander is tinkering with his mechanics this Spring Training to create more downward tilt on his fastball, which ranged from 93-96 mph on Friday. His three strikeouts came on fastballs that were swung on and missed.
"I speak highly, because you like his stuff and what he's capable of doing," Mattingly said. "He's one of those guys we feel like has a chance to be successful at this level, and also to really be able to help our club.
"He's got great stuff. If he can take that next step, he has a chance to be one of those top-of-the-rotation guys you can count on every time out. His stuff is good enough to beat anyone."
Joe Frisaro is a reporter for MLB.com. He writes a blog, called The Fish Pond. Follow him on Twitter @JoeFrisaro and listen to his podcast. This story was not subject to the approval of Major League Baseball or its clubs.