Allen hurt the anterior cruciate ligament in his left knee in the third quarter of the Dolphins' loss to New Orleans and will require surgery, coach Tony Sparano said Monday.
Sparano conceded he was unsure whether the rookies were ready to assume such a load.
"They need to mature really fast," Sparano said. "I don't know if they're equipped for 10 games in a row of doing full-time duty without having Will Allen out there. But these guys are playing at pretty good levels."
The loss of Allen depletes a secondary already under strain. Miami (2-4) has allowed 73 points in the past two games, with pass defense a big part of the problem. The Dolphins rank last in the NFL in yards allowed per completion.
Davis has seen significant playing time as a reserve and on special teams. He played 28 snaps Sunday against the Saints.
"I've been playing well and getting better and better," Davis said. "That's the biggest thing -- improving. I'm comfortable in the defense."
Smith won a starting job in training camp. Since April, he and Davis have been projected as the team's cornerback combo of the future.
"We knew this day was going to come somewhere down the road," Smith said. "We didn't expect it to be this soon. Now that it's here, we have to step up and rise to the occasion."
Allen, who has missed only one game since joining the Dolphins in 2006, leads the team with two interceptions this year.
"He's the field general," Smith said. "Before the play, he would give us alerts and things to look out for. For the most part, he's always right. Losing a guy like that is definitely going to hurt us."