Patrick Willis' retirement leaves a huge hole in the middle of the San Francisco 49ers' defense.

Seven Pro Bowls. Five All-Pro selections. Countless big plays with 950 tackles, 20.5 sacks and eight interceptions in eight seasons. And immeasurable moments as a team leader.

So where do the Niners go from here?

Back to leaning on the same guy who stepped up last year when Willis went down after just six games with that chronically injured left big toe -- Chris Borland.

But nothing the Niners do is easy, so that plan is contingent upon Borland being healthy himself because, remember, he missed the final two games with an ankle injury.

And at a generously listed 5-foot-11, 248 pounds and a playing style that borders on full-contact karate, you have to wonder how long Borland can sustain such a flair on the field.

While Borland is no Willis -- no one is -- the then-rookie stepped in quite admirably for the grizzled vet.

Borland racked up tackles like the Niners collected Lombardi Trophies in the 1980s and was all over the field in doing so with 107 stops, two interceptions and a sack in 14 games, eight of them starts.

The question, then, is can Borland sustain such success over 16 starts?

For the Niners to maintain the defensive success they enjoyed under former defensive coordinator Vic Fangio, new defensive coordinator Eric Mangini certainly hopes so. Especially with the defense primed to look more like it did in 2014 than it did in 2013, despite other expectations.

Look, Willis was expected to make a full recovery and be joined in the middle of the Niners’ 3-4 defense by another former All-Pro, NaVorro Bowman, who missed all of last season still recovering from the gruesome left knee injury he suffered in the NFC title game in January 2014.

But now speculation is rampant that Bowman could be cut, as well, a did-not-pass-the-physical casualty. Granted, it’s all conjecture at this point, but it’s also a logical leap.

It then seemingly pulls inside linebacker Michael Wilhoite -- who started 16 games at “jack” linebacker last season and had 87 tackles, two interceptions and a forced fumble -- off the reported trade market, per the Sacramento Bee.

“The standard that Patrick Willis and NaVorro Bowman have set for inside linebacker play is at the highest level,” former coach Jim Harbaugh said during the season. “Chris Borland, Michael Wilhoite, they are playing great, with a capital ‘G,’ and rising to that level. It is great to see.”

And then there’s the curious case of defensive lineman Justin Smith, who has yet to announce whether he is retiring or returning for a 15th NFL season. The Niners have, however, signed Darnell Dockett.

The linchpin to this whole Willis-retiring deal, though, is Borland.

“Dude can play football,” fellow linebacker Aaron Lynch said after Borland recorded 12 tackles, two for losses, plus a special-teams tackle, three passes defensed and two interceptions against the New York Giants in Week 11.

Replacing a potential Hall of Famer like Willis, Borland will have to maintain that, ahem, style of play for the Niners' defense to remain elite.