While missing the All-Pro to start the 2013 season isn't likely to doom the Patriots, his potential absence will create a hole in the New England offense that hasn't been easy to fill in the past and won't be easy to fill now.
According to Ian Rapoport of NFL.com (via Dan Hanzus), Gronkowski underwent successful surgery to repair a herniated disk in his back. The microscopic lumbar discectomy was performed by Dr. Robert Watkins, who completed a similar operation on New York Giants defensive end Jason Pierre-Paul earlier this offseason.
For the Patriots, the waiting game on Gronkowski continues.
The surgery has a typical recovery timeline of 12 weeks, putting Gronkowski in jeopardy of missing the start of next season. And even if Gronkowski's recovery lands on the early end of the timeline, the Patriots may still choose to hold out their star tight end a couple of extra weeks to be on the safe side.
Keep in mind, Gronkowski has dealt with four surgeries to his forearm since last November. Tuesday's back surgery represents his fifth operation in the span of just eight months.
Weighing all the factors, New England is likely to be dealing with life without Gronkowski for a period of time next season. The Patriots have struggled in those scenarios recently.
As pointed out by Chris Wesseling of NFL.com, quarterback Tom Brady had a sparkling passer rating of 106.6 and was widely considered an MVP candidate before losing Gronkowski last season. After Gronkowski's injury, Brady's passer rating plummeted to 87.7.
The statistical dropoff, of course, cannot be solely attributed to the loss of Gronkowski, and the Patriots still went 4-1 in the five regular-season games he missed. But the numbers are telling.
Despite possessing a league-high seven tight ends, the Patriots simply don't have a player on the roster capable of completely replacing what Gronkowski brings on every snap. While Aaron Hernandez is one of the game's most versatile tight ends, he isn't the same kind of red-zone threat or in-line blocker that Gronkowski so clearly is.
According to Pro Football Reference, Gronkowski's 29 touchdown receptions in the red zone are the most in the NFL since 2010. In fact, he has nine more than second-place Tony Gonzalez and Jimmy Graham, who each have 20 over the last three years.
There's little question that Gronkowski's 6'6" and 265-pound frame creates matchup nightmares near the end zone; linebackers aren't quick enough nor safeties big enough to handle him.
Such advantages against any kind of defender allow Gronkowski to make impressive TD receptions like this:
And a video montage of Gronkowski's touchdown catches over the last three years:
The Gronk's value, however, transcends just catching touchdown passes. The same frame that allows Gronkowski to dominate defenders in the scoring area also gives him an advantage as an in-line blocker.
According to Pro Football Focus (subscription required), Gronkowski has graded out as a top-five run blocker among tight ends in each of his three seasons, including a top finish in 2011. His combined grade of plus-36.3 since 2010 is the top mark at the position.
In the 10 games Gronkowski played and finished, the Patriots averaged 148.1 yards rushing. In the five games he missed, that number dropped to just 117.6.
Even when the Patriots have asked him to stay in and pass block, Gronkowski has been effective. In over 229 pass-blocking snaps since 2010, Gronkowski has allowed just 12 total pressures and a single sack.
In terms of in-line blocking, there's very little that Gronkowski can't do for the New England offense. Still, blocking may be one area where the Patriots can find a suitable replacement.
According to PFF (subscription required), Daniel Fells finished last season with an identical run-blocking grade as Gronkowski. He played a respectable sample size of 211 snaps in the run game, giving him ample experience to take on such a role to start 2013.
Fells wasn't the only replacement option in 2012. The Patriots also used veteran Michael Hoomanawanui, who finished with a positive grade in the passing game as an in-line blocker. New England can also expect to receive a contribution from former New York Giants tight end Jake Ballard, who missed all of the 2012 season with a torn ACL.
In his last full season with the Giants, Ballard played over 1,000 snaps and was a capable receiver and blocker. He caught 38 passes for 604 yards and four touchdowns as New York marched to the Super Bowl.
While not as athletic in the run game or accomplished as a red-zone receiver, Ballard can play a similar role in the New England offense while Gronkowski fully recovers. He's much more comparable in overall skill set to Gronkowski than either Fells or Hoomanawanui.
However, Gronkowski's ability as a safety blanket for Brady can't be understated.
The Patriots will enter next season with the likes of Danny Amendola, Julian Edelman, Michael Jenkins, Donald Jones and rookie Aaron Dobson leading the receiver depth chart. Amendola may have the capability of replicating Wes Welker's former role, but the receiving talent in New England is clearly limited.
Hernandez is an accomplished receiver, but his status is somewhat up in the air while dealing with an on-going homicide investigation, as reported by WBZ-TV’s Karen Anderson (h/t CBS Boston). And he, too, is also coming off shoulder surgery.
Without Gronkowski available, the Patriots will need several different members of the receiving corps and tight end group to grab hold of key roles in the passing game.
But even if the Patriots can get by with backups and an evolving set of pass-catchers, it's clear that the New England offense will miss the overall contributions of Gronkowski. His likely early absence has now created much uncertainty about what kind of offense we'll see from the Patriots to start 2013.