No plan B existed when the Broncos signed Peyton Manning in 2012. Does a plan C exist after Brock Osweiler signed with the Houston Texans?

In the span of three days, the Broncos lost their top two quarterbacks. Manning retired and Osweiler bolted in free agency, agreeing Wednesday to a four-year, $72-million deal with Houston with $37 million guaranteed.

Osweiler’s defection leaves the Broncos exploring all options for a starting quarterback, among them a trade for San Francisco’s Colin Kaepernick and attempting to sign veterans such as Ryan Fitzpatrick and Robert Griffin III.

Drafted and groomed as Manning’s replacement, Osweiler was an offseason priority after the Super Bowl 50 victory. The Broncos tried to keep him, offering more than $16 million per season over four seasons and in excess of $30 million guaranteed. Uncertainty, however, grew as the contract talks stalled. When the Broncos didn’t hear back for several days on their original offer and were unable to communicate directly with the quarterback, they believed Osweiler was looking for a fresh start outside of Manning’s shadow.

“We’ve stayed true to our philosophy of building a team with players who want to be Denver Broncos and want to be here,” general manager John Elway told the team’s website. “That’s been a successful approach for us. While we did offer a very competitive and fair longterm contract to Brock, we ultimately had to remain disciplined while continuing to assemble a roster that can compete for championships.”

The Broncos represent the first defending Super Bowl champion since the 2000 Baltimore Ravens to lose their starter and backup. Baltimore watched Trent Dilfer and Tony Banks sign elsewhere. Denver’s void is more striking because it involves Manning, the NFL’s alltime leader in touchdown passes and wins, and Osweiler, a 25-year-old entering his prime.

Denver made a strong push to keep Osweiler on Tuesday night and increased its bid on Wednesday as the Texans emerged as a viable threat. If not for Osweiler’s contributions last season, Denver would not have secured homefield advantage in the playoffs. Coach Gary Kubiak praised his mental toughness and poise.

Taking over on Nov. 22 at Chicago, Osweiler went 5-2 as a starter. That became an important statistic in the negotiations. Elway exercised restraint, unwilling to pay franchise quarterback money to a player with seven starts in four seasons.

“Our organization thanks Brock for his contributions and wishes him all the best,” Elway said.

Coach Gary Kubiak benched Osweiler in the season finale against the San Diego Chargers after the Broncos’ fifth turnover. Manning directed three consecutive scoring drives, never again relinquishing his spot. Teammates admitted Wednesday the demotion stung Osweiler more than he let on in January. The Texans swayed Osweiler with the most money and an opportunity to lead a contender. Houston won the AFC South before quarterback Brian Hoyer’s four-interception performance sabotaged them in a 30-0 home playoff loss to the Kansas City Chiefs.

Osweiler’s decision surprised former teammates.

“Shocked. I thought Brock wanted to play with us. Lol,” tweeted Denver cornerback Chris Harris.

Added Washington nose tackle Terrance Knighton, “(Darn). I didn’t think he would leave that defense.”

Elway began bracing for this outcome given the static nature of contract talks. Denver has lost defensive end Malik Jackson, linebacker Danny Trevathan and Osweiler in free agency. The departures create additional salary cap space to find a new quarterback. The Broncos hosted Kaepernick before the 2011 draft, weighing whether to choose him. His agents have asked San Francisco to trade him, though the 49ers have given no indication they will acquiesce. Fitzpatrick, who has the same agent as Osweiler, threw a career-high 31 touchdown passes past season with the New York Jets, but the 33-year-old has played on six teams in 11 seasons. Griffin won the NFL’s 2012 offensive rookie of the year honors. However, he hasn’t started since 2014, undermined by a knee injury and ineffectiveness.

Troy E. Renck: trenck@denverpost.com or @troyrenck