"We're gonna start Fitz this week. That's what we feel like we need to do to give ourselves the best chance," said Koetter, who would not commit to a starter beyond this week.
"This is just for right now. Right now. I mean, the most important thing we got is this week. We'll just have to see how things go. I don't like switching quarterbacks. That's not in my make-up to switch quarterbacks. But I just feel like we've got to make the switch at this time."
Koetter benched quarterback Jameis Winston on Sunday against the Cincinnati Bengals after the former No. 1 overall pick threw his fourth interception of the game, which was returned by safety Jessie Bates for a touchdown.
Fitzpatrick rallied the Buccaneers from an 18-point fourth-quarter deficit to tie the game, but Cincinnati won 37-34 on a field goal with no time remaining. Fitzpatrick went 11-for-15 for 194 yards and two touchdowns.
Turnovers have plagued Winston since college and played a big role in Koetter's decision. He has 11 turnovers in four games (three starts).
Winston has now thrown multiple interceptions in a game 16 times in his career, breaking a tie with Blake Bortles for the most multi-interception games since he entered the league in 2015.
"Jameis is a pro and he will continue to work on his craft. That's all he can do," Koetter said. "Very rarely is switching quarterbacks forever in this league. Just look around the league. It's very rarely forever. Jameis, I'm sure, will get another chance at some point, and when he does, he just has to do a better job of taking care of the football."
The Bucs already have picked up the fifth-year option on his contract, worth $20.9 million, but that is guaranteed for injury only. They could cut him with zero financial implications prior to the beginning of the new league year on March 13, 2019, at 4 p.m ET.
Fitzpatrick began the season as the team's starter while Winston served his three-game suspension for violating the NFL's personal conduct policy, stemming from an incident in which he allegedly groped a female Uber driver in Arizona in 2016.
Fitzpatrick has thrown for 1,550 yards with 13 touchdowns and five interceptions this season.
What does Fitzpatrick starting mean for Bucs, Winston?
Adam Schefter and Tim Hasselbeck take stock of Tampa Bay after Ryan Fitzpatrick was announced as the starting QB for Week 9 over Jameis Winston.