He'll likely need most, if not all of it, to recover after spraining his right foot in Sunday's 19-16 overtime win over the Bills.
"My foot was in the ground and someone was driving me backwards, and my foot couldn't get out of the ground, so my knee just got bent sideways," Roethlisberger said.
The Steelers (8-3) face the Ravens (8-3) in Baltimore next Sunday night with first place in the AFC North on the line. The Ravens beat the Steelers 17-14 in a Week 4 matchup at Heinz Field.
Baltimore held off the Buccaneers 17-10 on Sunday to improve to 5-0 at home.
The Ravens have won eight straight at M&T Bank Stadium dating to last season.
Roethlisberger didn't have his best game Sunday, completing 20 of 33 passes for 246 yards. But he led a late scoring drive in overtime and had an 18-yard scramble in the fourth quarter to set up a pair of big field goals.
"Luckily, our D-line got to him a lot," Bills linebacker Paul Posluszny said. "But he's a great quarterback and he's tough to bring down. He's a good athlete."
Information from ESPN.com's James Walker was used in this report.