I think first of all, ANY back surgery is "major" or serious, especially when it's your 3rd one. I mean, c'mon.
Secondly, "resting it out" (pitiful, terrible wording from Curran) and hoping that physical therapy helps the body to reabsorb the fluid/gel like material from the ruptured/herniated disk is pretty much a futile effort, unless one is willing to go through 18-24 months of sheer agony, or they get lucky and see results through steroid epidurals, etc. There's obviously a reason why the team didn't even bother attempting that route, which is almost always the preferred initial method of treatment. As you noted, he'd have been risking permanent nerve damage, so why "resting it out" would even be considered for him at this point in time is beyond me. Curran is wrong on this.
It also can take months and months before you have any hint as to whether or not the PT is even working, so if he did "rest it out" only to find out in the spring that he'd need surgery, a majority of 2017 could be in jeopardy. Much like JJ Watt, they bypassed any thought of PT and went right for surgery this time. Again, this is for a reason. The days of resting it out are gone in situations like this. I'm sure no one wished to concede the remainder of Gronk's 2016 production heading into the postseason because "it wasn't major..."
When it ruptures, the material inside leaks all over your nerve endings, often including symptoms of sciatica--which is what he experienced the first time. They then go in (lamenectomy, and often diskectomy) and clean up all of that exploded material to allow the nerves an attempt to rejuvenate. Sometimes it happens in 3 months, sometimes in 12 months, and sometimes not at all.
If you read his comments from the first one, he was quite concerned that he'd never regain feeling in his bottom legs/toes again, which is where I'm at after about 3 months out of surgery right now. Since that fluid inside the disk cannot come back and we've yet to see medical breakthroughs for "fake disks" implanted, there's no longer anything to hold the back up in place. It's why people often need a fusion surgery for their spinal column. Either way, post-NFL career, he will have issues for the rest of his life.
I would bet any amount of money that he will almost certainly need that fusion surgery at some point in the next handful of years, and had it not been for his football career, he'd have likely gotten it already. He's a freak athlete so he may be able to return to 75-80 percent of where he was at some point, but he may not be long for this game after this latest news. Fingers crossed.
Here's an article from his obstacles on the last one. They don't get
easier as they go on, but quite the opposite.
Patriots TE Rob Gronkowski can return6 from back surgery, but will he be same dominant player? - The Boston Globe