The Bills host the New England Patriots in their opener on Sept. 8.
If Tuel does indeed start for the Bills in Week 1, he will become the first undrafted rookie quarterback in the common draft era (since 1967) to start a season opener, according to the Elias Sports Bureau.
Manuel remains sidelined after undergoing a minor procedure on his left knee last week. Marrone said that while he hopes the 16th overall pick will be ready to play in Week 1, the team will make preparations for Tuel to start.
The Bills are proceeding as though Kolb's concussion, which was termed "serious," could be career-ending, a league source told ESPN.
Kolb will undergo a battery of tests, and the Bills do not want to rush him back even if he can continue his career, the source said.
ESPN NFL Insider Adam Schefter contributed to this report.