Allen will replace Brock Purdy (shoulder) as the starting quarterback Sunday at Green Bay, Adam Schefter of ESPN reports.

49ers coach Kyle Shanahan made the announcement Friday afternoon, revealing that the team was surprised by the degree of pain in Purdy's throwing shoulder when he attempted to practice Thursday. Purdy was listed as a limited participant for that session, sandwiched between absences Wednesday and Friday. 49ers GM John Lynch said Friday morning that MRIs on the shoulder didn't suggest a long-term injury, but it nonetheless seems possible at this point that Allen could make multiple starters. Beyond Sunday, the 49ers play at Buffalo in Week 13 (Dec. 1), at home against Chicago in Week 14 (Dec. 8) and at home against the Rams in Week 15 (Dec. 12, TNF). The game in Green Bay this Sunday will mark Allen's first NFL start since 2001 (with Cincinnati). He's gone 2-7 as a starter, with a career 56.7 completion percentage, 6.1 YPA, 10 TD passes and six INTs, adding 34 rush attempts for 63 yards and no TDs.