"Right now he's day to day, and whether his next start is in question, we'll probably have a better read on that once we get through the Tampa series," Farrell said Sunday.
The injury occurred in the sixth inning Saturday against the Angels when Howie Kendrick hit a line drive that knocked Buchholz's glove right off his hand. Buchholz left his glove on the ground and chased down the ball behind the mound. His off-balance throw to first was too late to get Kendrick.
"A little nagging there for a couple of innings," said Buchholz on Saturday night. "It was right after that ball that hit off my glove. When I threw it to first base, that's when it sort of tightened up a little bit. I figured that when I got the double-play ball and didn't turn it [in the seventh], I figured that was enough. I didn't want to push my luck."
Buchholz missed a start in late May due to irritation in his AC joint, which is located near the collarbone.
"It's close to that AC joint, so I can't say that they're not interrelated at some point, but it was more the result of that awkward throw that started to trigger it, and the cramping that followed," Farrell said.
Though he left the game in the seventh, Buchholz still picked up the win to move to 9-0 on the year. In 6 2/3 innings, he surrendered two runs and six hits while striking out four and walking one. His ERA is an MLB-best 1.71.
Michael Periatt is an associate reporter for MLB.com. Follow him on Twitter @Michael Periatt.