Lohse, who pitched five innings of one-run ball Monday, suffered the injury running to first base on a bunt attempt. His finger caught Padres second baseman Jedd Gyorko, who was covering first base on the bunt.
Lohse has registered a 2.52 ERA and 0.96 WHIP with 17 strikeouts in 25 innings so far this season, but has just one win. He's the Brewers' No. 2 starter behind Yovani Gallardo.
Lohse is the only pitcher on the Brewers' roster with at least four starts that has an ERA below 4.50. Gallardo has registered a 5.24 ERA in four starts, allowing 13 earned runs in 22.1 innings. Milwaukee ranked 22nd in ERA last season.
Right-hander Mike Fiers, who has allowed seven runs in 7.1 innings this season and was recently optioned to Triple-A, would likely be brought back up if Lohse was to miss any time.
The Brewers were 9-8 heading into Monday's game after going 83-79 last season. They ranked 19th in runs and 21st in team ERA.
Lohse went 16-3 with the St. Louis Cardinals last season. That included posting a 2.86 ERA and 1.09 WHIP. He agreed to a three-year, $33 million deal with the Brewers in late March. His absence would obviously be a big blow to a Brewers team that doesn't have a lot of talented arms.