When the Eagles signed Malcolm Jenkins, they touted his versatility and noted that the former Saints safety could cover receivers out of the slot.

It wasn't until training camp, however, that their plans for Jenkins came more into focus. They wanted enough flexibility that they could stay in their base defense even if offenses went with "11" (three wide receivers) personnel.

The Eagles employed this tactic for most of the Seahawks game, and while it allowed them to effectively slow running back Marshawn Lynch, it exposed Jenkins in the passing game.

It was a pick-your-poison proposition for defensive coordinator Bill Davis.

"That's why we have the safeties we have: so they cover that slot," Davis said this week. "Big running game out of [the Seahawks'] 11 personnel on the first and second down, the read option in particular. So we choose to stay in our base package and play the run."

But Nate Allen, Jenkins' counterpart, rarely covers receivers in the slot when the Eagles are in base. The Eagles' safeties are interchangeable in that both can play either near the line or in center field, depending upon the offensive alignment.

The Seahawks knew that Jenkins vs. receiver Doug Baldwin in the slot would give them an advantage, and they completed five passes for 78 yards and a touchdown when targeting the safety. As steady as Jenkins has been for most of the season, Sunday's game emphasized the difficulties of playing defensive back in the Eagles' scheme.

"I'm very comfortable with it. I don't think we've ever had a matchup where we felt like we couldn't run base," Jenkins said. "But that's a tough position to play. The slot's a lot tougher because you've got two-way goes, you've got a lot of space to cover."

Allen had his struggles, as well. He lost his assignment on a 20-yard pass that converted a third-and-15, and he broke containment when Lynch caught a 15-yard touchdown. Jenkins may have brought consistency to a position that has been problematic for the Eagles over the last five years, but safety still remains unsettled.

It seems implausible that Allen, who was signed to a one-year contract in March, will be back next year. Earl Wolff went to injured reserve, but he disappointed in his second season and it's unclear how much his knee affected his performance.

The Eagles have already begun looking ahead. They plucked rookie Jerome Couplin off the Lions' practice squad this week. He has good size (6-foot-2, 215 pounds), abnormally long arms (an 81-inch wingspan), and may be insurance should Wolff never return to form.

Couplin played in eight games for the Lions, but mostly on special teams. He said he played both free and strong safety in college at William and Mary, which could help his transition into the Eagles' defense.

The Eagles' scheme favors adaptable safeties, but they're hard to come by. The Seahawks have two of the very best safeties in the NFL, but Earl Thomas plays mostly free and Kam Chancellor plays mostly strong for a reason. It's difficult to be good at both.

Jenkins has the added responsibility of playing man-to-man defense in the slot, but what it has done is lessen nickel cornerback Brandon Boykin's role. He's played 41 percent of snaps this season, down 10 percent from last season.

Boykin was the Eagles' best defensive back in 2013 when he led the Eagles with six interceptions. He hasn't been as reliable this season. He has one interception. But the Eagles defense, overall, has been more consistent because the scheme around the strength of the unit - the defensive line.

The decision to go with base vs. "11" personnel involves many factors - opponent, down, distance - but it is indisputably more effective at slowing the run. The Eagles offense, for instance, averages 5.0 yards a rush in "11" and 3.6 yards in their "12" personnel (two tight ends).

But if teams throw on those downs, the Eagles will sometimes be at a disadvantage. Baldwin beat Jenkins for a 23-yard touchdown when he turned the safety around with a double move. Davis put Jenkins in a tough spot with a blitz, but that's a tough spot under any circumstance.

Among other things, Allen has had trouble plastering receivers when quarterbacks have bought time in the pocket.

"There really is no technique for it," Allen said. "Just find the guy and stay with him the best you can. It turns into sandlot football."

And there isn't a safety net.

jmclane@phillynews.com

@Jeff_McLane