"That's a quality lineup over there, and they are going to make you work," Koehler said. "If you don't execute pitches when you are ahead in the count, they can still put some runs on the board."
Koehler allowed three straight hits with one out in the second inning, but beared down and retired the next two batters to finish the frame.
"I thought the first time up for Tommy, the ball was coming out of his hand really well," Marlins manager Mike Redmond said. "He missed up in the zone just a touch, but he had a good rhythm and tempo going."
Before his next start, the righty is hoping to get a better feel for his changeup and other offspeed pitches.
"I threw a couple of changeups for strikes that were good, but no doubt in my next side session I need to be crisper. That will come," said Koehler.
Brad Hand, who's in competition with Koehler for the final spot in the rotation, also tossed two innings. He allowed one run on one hit, while walking one.
After retiring the Red Sox in order in the third inning, Hand struggled with his command in the fourth. He allowed a four-pitch leadoff walk to Shane Victorino, a double to Daniel Nava and fell behind 3-0 to Ryan Hanigan. However, he battled back after that, striking out Hanigan and then retiring the next two batters.
Redmond said before Friday's game that Hand will be built up like a starter.
"We'll try to get those guys as many innings as possible, so they are prepared in whatever role they have at the end," said Redmond.
Quinn Roberts is a reporter for MLB.com. Follow her on Twitter at @qlr5001. This story was not subject to the approval of Major League Baseball or its clubs.