The Cowboys' hope is that Romo will play only if Johnson is injured, a team source told ESPN's Ed Werder.
But the Cowboys are prepared to use Romo if the offense falters, as the team's main priority is to win the game, the source said.
Romo, who will not get a pain-killing injection, will wear a three-piece protective device over his broken finger, and the fractured portion of the finger is over the knuckle.
After Romo was hurt, the Cowboys anticipated he would be out a month. Johnson was set to make his first start since Dec. 17, 2006, for the Vikings.
St. Louis Rams offensive tackle Orlando Pace was inactive after injuring his thigh in the fourth quarter of last week's 19-17 victory at Washington. Adam Goldberg, who started two games at guard when Jacob Bell was out with a foot injury earlier in the season, started in place of Pace.
Rams wide receiver Dane Looker, who returned to practice this week after being sidelined with a brain abnormality, was inactive.
Information from The Associated Press was used in this report.