It was unclear as to how and where exactly Barrett injured himself. Williams noticed that Barrett was uncomfortable on the mound, prompting the move to pull him from the game.
If Barrett misses time, it would be yet another blow to a team that is beginning to be plagued by injuries. Starting first baseman Ryan Zimmerman was placed on the 15-day disabled list earlier Thursday.
Barrett entered the game having struck out 27 batters and walking just five over 20 1/3 innings this season. He ranks second among N.L. relievers in K/9.0 IP with a mark of 11.76.
He was charged with one run on one hit and walk, striking out one, on Thursday. Barrett took the loss, falling to 3-2 on the season.
Brandon Curry is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.