The best NFL general managers know when to hold ‘em and know when to fold ‘em.

They are smart enough to know a player’s sell-by date and coldhearted enough to part with them before their trade value starts to nosedive.

That’s what has happened with the Eagles and their Pro Bowl tight end, Zach Ertz. General manager Howie Roseman made the mistake of waiting a year too long to trade Ertz, and now Roseman is going to have to settle for far less for Ertz than he could have received 12 months ago.

Last year at this time, the Eagles could have gotten a third-, maybe even a second-round pick for Ertz. At 29, he still was in the same tight end conversation with Travis Kelce and George Kittle. Not as good as them, but certainly in the same zip code.

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His 88 catches in 2019 were the third most in the league among tight ends, behind only Kelce and the Raiders’ Darren Waller. A year earlier, he had broken the NFL record for receptions by a tight end with 116.

But that was then and this is now, and now Ertz is coming off an injury-plagued season in which he missed five games and caught a career-low 36 passes and one touchdown. He had a career-low catch rate (50%) and yards-per-target average (4.7) and had the third lowest yards-after-catch average among all NFL tight ends (2.8).

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In his last five games after returning to the lineup in early December, he had just 12 catches on 27 targets. No touchdowns. Just three third-down receptions. Zero red-zone catches.

As the retooling Eagles look to the future and try to get younger and add as many draft picks as possible, they have made it clear to the rest of the league that Ertz is available.

They’ve received offers, but nothing to write home about. Sixth- and seventh-round picks. The Eagles already has five of those that they’re going to throw against the wall in next month’s draft. They really need something more substantial in exchange for a guy who caught 204 passes in 2018-19. But that’s what happens when you wait too long.

Roseman has given Ertz’s agent, Steve Caric, the green light to talk to other teams about a deal. But he’s not likely to squeeze a better offer out of anyone than Roseman was.

“Zach’s been a huge part of the success that we’ve had since we drafted him,” Roseman said Thursday on a Zoom call with reporters and anyone else with an internet connection who wanted to listen in on the team’s website. “He is a heck of a player, he is a heck of a person and we value him. For us to trade any player, it’s got to make sense for both sides.”

With the emergence of the Eagles’ other tight end, Dallas Goedert, Roseman knew last year that he probably wasn’t going to be able to keep both Goedert and Ertz together long-term.

A few teams had expressed interest in Ertz after the 2019 season. But when the pandemic hit, Roseman decided to put the team’s planned rebuild on hold for a season.

“The biggest regret I have is knowing we were an older team going into last offseason,” Roseman said last month. “Knowing there was a lot to do and this was the time to do it.

“And then when the pandemic hit, I’m thinking, no, no, no. This is the moment to make another run, go win the division, go back to the playoffs because the other teams in our division had new coaches and were transitioning.”

We all know how that worked out.

Roseman tried to sign Ertz to a modest extension before the start of last season that would keep him and Goedert together through 2022. But the negotiations not only weren’t fruitful, they also became acrimonious. You probably can add that to the list of reasons Carson Wentz wanted out, given his close relationship with Ertz.

Ertz decided to bet on himself last season to have another Pro Bowl year, but that panned out as well as Roseman’s playoff hopes for his team.

“You talk exactly about the dilemma you have right there [with Ertz],” Roseman said. “This is a guy we didn’t want to trade last year because we wanted him to finish his career as an Eagle.

“When we looked at the history of tight ends in the league, you’re talking about a 30-year-old guy, not somebody in his mid-30s. And his skill set, we felt like he had tremendous production going forward left in him. We still feel that.

“There’s nobody that works harder. He’s an incredible route runner. He’s got incredible hands. He’ll do anything to help the team.

“Our goal [last August] really was to extend him. That didn’t work out. We hadn’t really thought about trading him. We were trying to do whatever we could to win. And we knew he was a big part of that.”

Roseman mentioned the Eagles’ recent decisions to extend the contracts of 33-year-old center Jason Kelce and 32-year-old defensive end Brandon Graham even as the team enters what owner Jeffrey Lurie has described as “a real transition period.”

The Eagles’ GM said it was important to keep both of those players because they are “culture-setters.”

Not so long ago, many people might have also considered Ertz a culture-setter.

Turns out all he is is expendable.