Romo broke his right pinkie on the first play of overtime in Sunday's 30-24 loss at Arizona. Coach Wade Phillips said Romo, who had difficulty gripping the ball and throwing after the injury, would not have surgery.
"He couldn't grip the ball," said Phillips. "He probably should have gone out of the game. He had sprained his finger before, and thought that's what it was when he couldn't grip the ball."
When asked specifically how long Romo would be out, Phillips wouldn't commit to a timeframe, desribing it as a "week to week" injury and saying it depended on how fast the quarterback heels. He indicated the Cowboys won't sign another quarterback.
"Obviously, it's unfortunate for Tony to have to go through something like this. He'll recover quick, he has a great attitude about it," Johnson said. "At this time, it's really just important for the offense especially to just kind of catch on to my snap count, and the way we manage the huddle. ... I'm excited about the opportunity, worked hard to get here."
Phillips added: "We have complete confidence in him. Our team, I think, will rally around him. ... Brad had a good preseason. He's ready to play."
Neither Romo nor Johnson was in the locker room Monday when it was open to reporters.
When linebacker Greg Ellis was told about Romo's broken finger, he got a somewhat stunned looked on his face and exclaimed, "Wow!" Bradie James said the players will have to rally around each other.
"We've got to play relentless football now without our signal caller, our leader, our captain on the offensive side," James said. "That's the thing about a season; you never know what can happen. You hope you are playing some real good ball when things go down, so here we are and now we've got to make some adjustments and move on."
Receiver Terrell Owens refused comment, getting agitated when he saw cameras recording him. "Today is not Wednesday," he said, referring to the day he routinely talks to reporters.
Romo missed on the next two passes after the injury -- both intended for Owens -- but before then passed for 321 yards and three touchdowns despite heavy pressure. He was sacked three times and knocked down 19 times.
"Obviously, he had trouble throwing the ball that last series in overtime," Phillips said.
Romo, whose hand was heavily wrapped after the game, fumbled three times Sunday, losing one, but didn't throw an interception for the first time in the last nine games.
Through Sunday's games, Romo ranks second in the NFC with a 103.5 passer rating. He has thrown for 14 touchdowns and been intercepted five times. He had his team-record 14th career 300-yard passing game Sunday in his 32nd start. That broke a tie with Troy Aikman, whose 13 were accomplished in 165 starts.
In 2007, his first full season as a starter, Romo led the Cowboys to a 13-3 record, best in the NFC. He set team records with 335 completions for 4,211 yards and 36 touchdowns, including the team-record 15 receiving by Terrell Owens, in a season that ended with a loss to the Giants in the playoffs.
It was two years ago this month that Romo replaced Drew Bledsoe as the starter.
Johnson quarterbacked the Tampa Bay Buccaneers to the NFL championship in 2002. He has been Romo's backup since last season and threw only 11 passes in 2007.
Johnson, in his second year as Cowboys backup, has only seen brief mopup duty in Dallas, going 7-for-11 for 79 yards. Before coming to Dallas, Johnson started 122 of his 145 career games with Minnesota (1993-98, 2005-06), Washington (1999-2000) and Tampa Bay (2001-04).
Johnson has completed 2,627 of 4,248 career passes (61.8 percent) for 28,627 yards with 164 touchdowns and 117 interceptions. He passed for 2,750 yards in 2006 with nine touchdowns and 15 interceptions -- only the third time in his career with more interceptions than TDs.
"It's going to be Brad to go in there and prove that he's not just here for the ride, he's part of this thing," Ellis said. "Every opportunity I've seen Brad step in on other teams, he's able to hold it down and get it done."
Another former Viking, Brooks Bollinger, will be Johnson's backup.
The Cowboys also sustained other injuries Sunday.
Rookie running back Felix Jones strained his hamstring and was having and MRI on Monday. Punter Mat McBriar, who was carted off the field after his kick was blocked on the game's final play, also was having an MRI. The team doesn't know the extent of his injury yet, Phillips said.
The Associated Press contributed to this report