It looked bad at the time, as Lackey winced with a cramp, and his arm seemed to be dangling as he left the game.
But an MRI performed on Sunday in Boston revealed no structural damage.
"He's improved," said Farrell. "He's still in a 48-to-72-hour recovery period. There's no roster move to announce of any kind right now. The most encouraging thing is that following the MRI, it showed some inflammation in the biceps. That's been it. He's set for re-exam today. After we get that information, we'll map out our plan of attack going forward."
The Red Sox have an off-day on Tuesday, which means they don't need a fifth starter until Sunday.
Alfredo Aceves would be the likely choice for that day, assuming Lackey doesn't recover by then.
Lackey could be pushed back a couple of days in the rotation or be skipped entirely.
The Sox won't place Lackey on the disabled list until they know for sure he's going to need a couple of weeks to recover.
"When a pitcher walks off the mound in the middle of an inning, we're going to do what's in his best interest and not risk anything health-wise," Farrell said. "Before he gets back on the mound, we'll have to put him through a good test of some sort, whether that's an extended [bullpen session], a [simulated] game -- that will be taken into account."
Ian Browne is a reporter for MLB.com. Read his blog, Brownie Points, and follow him on Twitter @IanMBrowne. Jason Mastrodonato is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.