PHILADELPHIA -- They decided to change position coaches the day after the 2015 season. They spent money to bring in two veteran starters in free agency. They planned a training camp competition with 16 players, including seven who were in at least their fifth NFL season.

The Minnesota Vikings attacked their offensive line issues with their trademark thoroughness and diligence. And yet through a rash of injuries and a handful of subpar performances, the problem is as evident as ever. It coursed through the Vikings' first loss of the season, a 21-10 defeat in Philadelphia on Sunday, and in a year when the Vikings have been at their most resourceful, the ailment they can't shake could be the one that matters most.

The Eagles' Rodney McLeod forces a fumble from Vikings quarterback Sam Bradford with the Eagles' Mychal Kendricks also applying pressure. Rich Schultz/Getty Images

An offense that hadn't lost the ball in its first 57 drives this season turned it over three times in a span of six possessions on Sunday, with all three lost possessions attributable in some way to issues up front. Sam Bradford's first-quarter throw to the end zone fluttered as Brandon Graham drilled him after shoving aside T.J. Clemmings, and Rodney McLeod picked off the throw to Adam Thielen in the end zone. Bradford's first fumble came when Connor Barwin hit his hand at the end of a rush around Jake Long, and his second came after the Eagles blitzed Mychal Kendricks and McLeod to the left side of the line, getting past Ronnie Hillman and Long to drill the quarterback. He'd get stripped twice more late in the fourth quarter, though the Vikings recovered both.

"It's tough to evaluate [Bradford's] performance when you look like a sieve," coach Mike Zimmer said. "They didn’t block anybody. We were soft, got overpowered. It was a little bit of man-on-man and we got whipped."

And when the Vikings had a chance to pull within a score early in the fourth quarter, they turned to their offensive line on a pair of short-yardage situations inside the Eagles' 10. Zach Line was stuffed on third down, and Matt Asiata ran into a pile of bodies on fourth down as the Vikings came away with no points inside the 10-yard line for the second time on Sunday.

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The Vikings tried a three-tackle rotation between Long, Clemmings and Jeremiah Sirles on Sunday, and a team known for its meticulousness will keep looking for solutions. The problem is, there might not be many obvious ones. Starting-caliber tackles aren't exactly sitting idle by the phone, and even if they were, the Vikings don't have much financial flexibility. They had just $591,491 remaining under the cap after signing Long, due in large part to more than $30 million in cap space sitting on injured reserve, and any adjustments they make the rest of the way will require some tough financial maneuvers, with so much of the Vikings' cap space tied up in injured (and thus financially immobile) players.

It's possible Long will improve after shaking off some rust, and the Vikings have been able to get around their line issues in some weeks, thanks to Bradford's quick release. They faced a strong pass rush on Sunday, and a team that knew better than most how to affect Bradford. They weren't likely to go undefeated, and they'll have eight days to prepare for a Monday night matchup in Chicago.

But the Vikings' biggest issue has been more apparent as injuries have sapped their depth, and they don't have an obvious recourse other than to fix things with the players they have. Quite often in Vikings history, the best teams have been felled by a banana peel nobody saw. This year's obstacle, in some ways, appears to be standing right in front of them.

"We’re not going to go down the street and pick up a bunch of guys," Zimmer said. "We need to get these guys better and do it quickly."