Cory Undlin, the Eagles coach who did the least with the most, has no greater ally than Malcolm Jenkins. And Malcolm Jenkins is a good ally to have.

Undlin, 48, will be the Detroit Lions’ next defensive coordinator, his first coordinator’s job since he left his alma mater, California Lutheran, in 2002. He coached the Eagles’ defensive backs since 2015. None has flourished. Few have played to their pedigree.

Jenkins said Undlin deserves credit nonetheless.

“For him to do what he’s done with these guys, in the scheme we play, was really remarkable,” Jenkins told me last week. “He’s a very good teacher.”

That’s a hard thing to prove.

Jenkins spoke last Monday, well before Undlin got the Lions job a week later. At that point, it seemed more likely that Undlin wouldn’t have any job a week later. He seems to coach under a lucky star.

Undlin arrived in Philadelphia in 2015, Chip Kelly’s final season. That began Undlin’s association with a string of ineffectual Eagles cornerbacks. In his first season, Undlin got little out of $63 million free agent Byron Maxwell and second-round rookie Eric Rowe, both of whom were traded after that season, when head coach Doug Pederson arrived and retained Undlin.

The Eagles remade their cornerback corps the next season, in 2017, when they traded for Ronald Darby, drafted Sidney Jones in the second round (a first-round talent whose stock fell due to injury), and drafted Rasul Douglas in the third. Avonte Maddox arrived in the fourth round in 2018.

Darby, who missed significant time the last three seasons, never produced as expected when healthy. Maddox has had his moments, but he hadn’t proved dependable. Worse, Jones and Douglas did not play any defensive snaps at all in last week’s playoff loss to Seattle. Second- and third-round picks in their third seasons played only on special teams in the Eagles’ biggest game of the season? That’s the blackest mark in Undlin’s file. Position coaches are supposed to develop young talent. Period.

But the NFL is all about relationships, and Undlin spent 2004 on Bill Belichick’s staff in New England with current Lions head coach Matt Patricia. Eagles defensive coordinator Jim Schwartz, a former Lions head coach, recommended Undlin for the job. As a result, Undlin is now Detroit’s problem. Like fired offensive coordinator Mike Groh and fired receivers coach Carson Walch, consider Undlin’s departure addition by subtraction.

Granted, Undlin’s defensive backs got hurt a lot. Granted, he helped a handful of fringe players become viable NFL corners. But, after five years, you can’t hang your hat on the mediocre play of Cre’von LeBlanc and Jalen Mills.

The numbers never lie

The Eagles ranked 19th against the pass in 2019, and opposing passers had a 90.8 rating, also 19th-best in the league. Only the Raiders gave up more than the Eagles’ 15 passing plays of 40 yards or more in 2019. Only the Chiefs and Giants surrendered more passing plays of 20 yards or more in 2018. The Eagles intercepted just 21 passes the past two seasons combined, tied for fifth-worst in the NFL and fewer picks than New England had in 2019 (25) and Chicago had in 2018 (27).

Undlin’s signature moments were more ignominious than impressive. Remember the 17-point, fourth-quarter lead the Eagles blew against Carolina on Oct. 21, 2018? You know, the game in which the defense began the fourth quarter dancing? How about the 14-point lead they blew against woeful Miami on Dec. 1 in Game 12 this season — a game in which they gave up 23 points in the last 25 minutes?

Jenkins acknowledged the shortcomings of the defensive backfield, but he insisted that Undlin wasn’t the problem. He said Undlin did a good job considering the injuries, the inexperience, and the extensive skill sets required to be an effective corner in Jim Schwartz’s scheme.

However, in my conversations with several former defensive players and a handful of current defensive coaches the past two seasons, the consensus has been that Eagles cornerbacks struggle with proper techniques and lack predatory instincts. Schwartz’s defense is predicated on its linemen pressuring the quarterback into making bad decisions, but that can take too much time for the Eagles’ corners to cover receivers. The Eagles’ corners don’t excel in press coverage; they don’t ball-hawk in zone coverage, and they don’t win one-on-one battles with athletic wideouts.

So what, exactly, do they do well?

They’re OK in intermediate coverages. They tackle. Which is pretty much what they did two seasons ago.

New face in a new place

There’s a fair chance that Undlin will succeed in Detroit, where Patricia will run the defense and call the plays. Plenty of coaches turn out to be better coordinators or head coaches than they were position coaches, anyway. That was the case both with Andy Reid and Doug Pederson.

That isn’t always true, of course. John DeFilippo was a superior quarterbacks coach, and developed Carson Wentz with a firm and steady hand, but Flip failed as an offensive coordinator in Minnesota and Jacksonville the past two seasons. Undlin isn’t Flip, a hard-driving, sometimes abrasive perfectionist. Undlin is a pleasant, patient Minnesota native whose first job out of college was teaching the fourth grade. There’s a lot to be said for emotional intelligence, and it wouldn’t be surprising to know that the Lions’ offices didn’t get much of it last season from Patricia and Paul Pasqualoni, the coordinator Undlin will replace.

Jenkins usually knows what he’s talking about, and Jenkins is the only Eagles defensive back to reach the Pro Bowl since Undlin arrived, and he’s the only first-round talent Undlin coached in that period.

So maybe Undlin did make the most of what he had.

With all due respect to Jenkins, it seems like he made the least.