The agent described the growth as "muscular" and said it lacked "cancerous intentions." Still, it created irritation that Strasburg had to deal with during the season.
"It was bothering him when he pitched," Boras said. "He's fine now."
It's possible the issue could help explain some of Strasburg's injury problems this season.
The 27-year-old also tweaked his ankle during Spring Training, which might have caused him to alter his mechanics. He began the season in the rotation but posted a 6.55 ERA over 10 starts before going on the disabled list with a left trapezius strain.
Strasburg returned after a little less than a month but pitched only three times before another DL stint, this time for a strained left oblique. He also skipped a start after experiencing upper back tightness on Aug. 30 against the Marlins, though he was back in action on Sept. 9 and pitched five more times before the end of the season.
Despite the health issues, Strasburg went 8-2 with a 1.76 ERA over 13 starts following his first trip to the DL, striking out 110 batters in 82 innings. In 23 outings overall, he was 11-7 with a 3.46 ERA.
Strasburg, who will go through arbitration for the final time this offseason, is due to reach free agency at the end of the 2016 season. His name has popped up in some trade rumors, but Boras said he didn't know if the Nationals would consider moving the right-hander.
"Stephen Strasburg is a great pitcher," Boras said. "I don't know how you would begin to replace him in the market without it being very, very costly."
Andrew Simon is a reporter for MLB.com. Follow him on Twitter @AndrewSimonMLB. This story was not subject to the approval of Major League Baseball or its clubs.