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Finding All-Pro tight end Rob Gronkowski is proving to be easier than finding his backup.

The New England Patriots have been searching for an understudy to Gronkowski for the better part of the past four years, and they have come away with mixed results. From veterans Daniel Fells, Michael Hoomanawanui and Scott Chandler to rookies A.J. Derby and Justin Jones, the Patriots have tried a mix of players young and old.

Some have been stopgaps for the backup spot, others have been cast off before they ever had a chance to prove or disprove their talent, but none have stuck around for more than a few years before their time with the Patriots came to an end.

And now that the Patriots have released Chandler, the spotlight once again shifts to the backup tight end spot—all it takes is one more injury to Gronkowski for a backup to suddenly become a starter.

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The Patriots already showed interest in one tight end, Clay Harbor of the Jacksonville Jaguars, according to ESPN.com's Adam Schefter and Adam Caplan (h/t Mike Reiss of ESPN.com). Harbor backed up Brent Celek for three years with the Philadelphia Eagles, and Marcedes Lewis for three years with the Jacksonville Jaguars. However, he has career highs of just 26 receptions, 292 yards, and two touchdowns.

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At 6'3" and 250 pounds, he has enough size to go up against smaller defensive ends and linebackers, but he's a pass-catcher first and foremost—over the past three years, he earned 49.9 percent of his playing time while running routes, according to Pro Football Focus.

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There are more proven options out there, like Jermaine Gresham of the Arizona Cardinals. The Cincinnati Bengals' former first-round pick has had some very productive seasons at the tight end position, with an average of 56 receptions for 544 yards and five touchdowns in his five years with the Bengals.

He had by far the worst season of his career with the Cardinals, with just 18 receptions for 223 yards and one touchdown, but the Cardinals used him much less as a receiver (36.3 percent of his playing time) than the Bengals (between 42 and 50 percent of his playing time each year).

That being said, when it comes to judging a backup tight end—especially a backup to Gronkowski—a lot more has to go into the analysis than just statistics. After all, Gronkowski has been as dominant a blocker as pass-catcher throughout his career; he was Pro Football Focus' highest-graded run-blocking tight end in 2015, 2011 and even as a rookie in 2010; and he ranked in the top 10 in 2012.

Ideally, the Patriots could find one tight end that could step in and fill Gronkowski's massive shoes. The problem is, you're never going to find a singular tight end that will do everything that Gronkowski can do; backing him up has to be a committee approach. In the event of an injury, there will be a dropoff no matter what they do

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Looking for a well-versed pass-catching tight end? Gresham is one option. So is former San Francisco 49ers tight end Vernon Davis, although he's no longer the dominant and productive tight end he was for the better part of a decade in the Bay Area. He was traded midseason to the Denver Broncos, and he just wrapped up his first season ever without a touchdown.

Then, of course, there's the possibility that the Patriots go young with their backup tight end spot. For years, they've been trying (and failing) to find backups at tight end through the draft and undrafted free agency. Justin Jones and Asa Watson were failed experiments during the 2014 offseason, but A.J. Derby is still on the roster after spending all of 2015 on injured reserve.

Perhaps the draft might be the best way to go, and if the Patriots decide that's the case, Bill Belichick could ask his good friend Urban Meyer for a review of Ohio State tight end Nick Vannett.

The 6'6", 257-pound tight end has a large wingspan and the potential to become a great blocker with good coaching. It would take a second- or third-round pick to get him, but maybe it's time to put some resources into that position and try to solve the problem for the long term.

And let's make no mistake; the Patriots have Gronkowski under contract until 2020, so he's going to remain a central figure of the offense. Whether he continues to dominate the NFL or succumbs to injuries, the Patriots will not regret prioritizing a backup.

Do they have any regrets about prioritizing the backup quarterback spot? Jimmy Garoppolo has earned far less playing time than a backup tight end would earn, even if Gronkowski remains healthy.

Over the past two years, the Patriots have been much more fortunate with Gronkowski's health than they were in his first few seasons in the NFL. They can't bank on fortune, though, so something must be done about their paper-thin depth at the tight end position.

Unless otherwise noted, all advanced statistics obtained via ProFootballFocus.com.