The Vikings, who still believe a Lombardi Trophy is within their grasp this season, have moved swiftly and boldly to replace Teddy Bridgewater.

Four days after their starting quarterback was lost to a season-ending knee injury, the reigning NFC North champs on Saturday acquired Sam Bradford in a stunning trade with the Philadelphia Eagles.

“Very rarely do you have a starting quarterback go down a week before the season,” General Manager Rick Spielman said. “And I think as we went through this process and spoke with all the teams … we felt this was a very rare and unique opportunity to add a quarterback of Sam’s caliber.”

During an injury-prone six-year NFL career, Bradford has not lived up to the billing of being the top pick in the 2010 draft. But the 28-year-old, who has now been traded twice in the past 18 months, is a clear upgrade over Shaun Hill, the 36-year-old who was slated to be Bridgewater’s top backup in 2016.

The addition of Bradford, who is under contract through 2017, gives the Vikings a starting-caliber quarterback for this season and an insurance policy in the event that Bridgewater is not ready to return from his devastating injury by the start of next season.

But the Vikings had to pay a steep price to acquire him.

Before Thursday night’s preseason finale, Spielman insisted the Vikings were not desperate and said they would not make a move that would “hurt us going into the future.” Less than 48 hours later, he traded their 2017 first-round pick and a conditional pick in 2018 that can become a second-rounder to get Bradford.

“We did not mortgage the future in my mind,” Spielman said after the trade, pointing out that the Vikings still have eight 2017 draft picks.

The conditional pick in the deal is at minimum a fourth-round pick. If the Vikings win the Super Bowl with Bradford playing at least 80 percent of their snaps, it will reportedly become a second-rounder — and it would definitely be worth it.

But Bradford has a 25-37-1 career record and has never started a playoff game.

Then again, he never was surrounded by a stacked supporting cast like this — with All-Pro running back Adrian Peterson and a talented, young defense — in his five seasons with the St. Louis Rams and his lone season in Philadelphia.

“Obviously, it’s a very talented football team,” Bradford said. “I’m excited to be a part of this football team. And I’m really just trying to familiarize myself with both the personnel and the playbook right now.”

After an underwhelming start in St. Louis during which he averaged 6.7 yards per attempt or less in his first three NFL seasons, Bradford threw 14 touchdowns and only four interceptions in seven 2013 starts before tearing the anterior cruciate ligament in his left knee. The following preseason, he tore that ACL again.

The man who replaced him to start that 2014 season? None other than Hill.

The Rams traded Bradford to the Eagles in the 2015 offseason. The Eagles flopped in coach Chip Kelly’s final year in Philadelphia. But Bradford set career highs in passing yards (3,725), completion percentage (.650) and yards per attempt (7.0).

The Eagles, under new leadership in 2016, signed Bradford to a two-year, $36 million contract in March. But they also added veteran backup Chase Daniel and drafted North Dakota State’s Carson Wentz with the second overall pick in the 2016 NFL draft. Philadelphia reportedly plans to start Wentz, making Bradford expendable.

Bradford said the first he heard about potentially being traded was when Eagles coach Doug Pederson called him after the deal had already been finalized.

“I was a little surprised just because it wasn’t on my radar,” he said. “That quickly turned into excitement when I realized the opportunity that I had ahead of me.”

Bradford got that call around 8 a.m. The Vikings sent a private jet to Oklahoma City, and the quarterback was in the building at Winter Park by late afternoon. He passed a physical, met the coaching staff and got to work on the playbook.

New tight ends coach Pat Shurmur — who was the Eagles’ offensive coordinator last season and also Bradford’s offensive coordinator in his rookie year with the Rams — told the Vikings that he could pick up Norv Turner’s offense quickly.

The Vikings accepted Bradford’s deal as is, so he is set to earn a $7 million base salary in 2016. While Bradford is under contract through the 2017, the Vikings can move on from him without cost by releasing or trading him before the fifth day of the 2017 league year in March, when he is due a $4 million roster bonus.

That contractual flexibility is critical for the Vikings, who won’t know the full extent of the damage to Bridgewater’s left knee until he undergoes surgery later this month. In addition to a dislocated knee and a torn ACL, he suffered other structural damage that could keep him out past the start of the 2017 season.

“I think everybody knows how we feel about Teddy,” Spielman said. “Going into next year, [we have two] very talented quarterbacks.”

The plan remains for Hill to start the season opener against the Tennessee Titans since Bradford  practiced with the Vikings for the first time Sunday and will have five total practices at Winter Park before next Sunday. Bradford could be ready to start by the home opener against the Green Bay Packers on Sept. 18.

“I’m going to work as hard to get myself ready, and whenever the coaches feel like I’m ready and able to help this team, I’m sure they’ll make the decision,” he said.

At Oklahoma, Bradford won the Heisman Trophy in 2008 and was a redshirt freshman during Peterson’s final college season. Now the Vikings hope the two former Sooners can spark a potent enough offense to help them — sans Bridgewater — get back to the playoffs.

“We had to make a very difficult decision,” Spielman said. “I know by adding Sam Bradford to this roster, we are a better team today than we were yesterday.”

Staff writers Andrew Krammer and Mark Craig contributed to this story.