Lugo severely strained the quad on July 11 while running out a grounder and had recently made significant progress.
On Monday, Lugo took batting practice for the first time since suffering the injury and even speculated he could go on a Minor League rehab assignment at some point next week.
But that all changed Wednesday.
Lugo had the quad wrapped a couple of hours before Wednesday's game and was noticeably limping.
"Not good," Lugo said. "I pulled it again. I was running outside today and it was hurting. I'll have to shut it down and see how it feels tomorrow."
How serious is it?
"I'm hurting right now," said Lugo.
How did it compare to the pain he felt when he suffered the original injury?
"This is not as much, but this muscle was hurt before," Lugo said. "Something like this can hurt again and set me back. Now, I've had a setback."
The good news for the Red Sox is that Jed Lowrie has been a key contributor on offense and a steadying presence on defense while filling in for Lugo.
The rookie shortstop is hitting .314 with one homer and 30 RBIs. Lowrie hasn't made an error in the 46 games he's played this season.
In 261 at-bats, Lugo is hitting .268 with one homer and 22 RBIs while making 16 errors.
Ian Browne is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.