Paul Lewis: Who is the All Blacks' next best number eight?

Maybe Kieran Read's back surgery is the answer, rather than the question. It may tell us why the All Black skipper has struggled for form at the end of the last two seasons - or, rather, the sort of form for which he is known.

It may also tell us who is best suited to take over from him at No8.

Read, even at 34 (as he will be in 2019), is the No1 choice at No8 for the World Cup in Japan.

Part of that is because, well, he's Kieran Read. In addition, in spite of rampant depth-building (55 pulled on the national shirt this season), the All Blacks are a bit thin at No8 should Read be absent for any reason.

There have been quite a few reasons lately. He missed the test against Wales on the November tour because of a hip problem stemming from his back. He has had a broken thumb and head knocks and, while you can't say Read is off form, he hasn't consistently risen to former heights.

His trademark is running wide, pace and power, an effective link and distributor, armed with deft offloads. He was the All Blacks' third most prolific tackler this season - behind the two Sams, Whitelock and Cane. Then there's that engine; we are talking Richie McCaw and Brodie Retallick-type power plants, revving high even when the tank is dry.

But at the end of the last tour - and that of the previous year - Read looked battle-weary; even the All Black coaches described him as "battered". The free-ranging flier often became a grunt work grafter - also a reflection of the pressure from defences committed to upsetting the All Blacks' rhythms.

This is the midway point in the World Cup cycle, rugby's Holy Grail. Teams and individuals can't peak all the time and, at 32, it gets harder, much harder, to do so.

A back injury is also a potential game-changer for any athlete, let alone a player exposed to rugby's collisions and weird angles a frame is contorted into in tight-loose exchanges such as clear-outs.

Cotton wool may keep him fresh but what happens if Read doesn't make it back?

Liam Squire is best qualified but has done so well at blindside the ABs' selectors will be loath to shift him, depending on Vaea Fifita's development. There is no doubt about his raw talent but the "top two inches" element has yet to be proven, as is his effectiveness at the breakdown.

Akira Ioane is potentially the most exciting prospect but, while his open play is unquestionably good, there remain questions about his close-quarters stuff.

Luke Whitelock, who subbed for Read against Wales, is a crunching defender but does not yet have the attacking skills of the abovementioned nor those of Brad Shields. It was he to whom the All Blacks originally turned when Jerome Kaino was injured on tour, only to discover Shields had signed to play in Britain.

Small wonder. Shields, a talented player, had watched as Squire, Fifita, Steven Luatua, Elliot Dixon and Ioane were all promoted ahead of him. There was a message in the roaring silence. His departure means there could be more depth-sounding next year - possibly involving Jordan Taufua and/or Dixon.

Taufua seems to have been around for ever but is only 25 - and has been passed over before, never mind his aggressive hunting-gathering and a strong fan base. His main obstacle is his size and he is not a lineout option in an era when the bench has to be carefully calculated, sometimes involving loose forwards who can double at lock.

Taufua was also relatively quiet when the British and Irish Lions beat the Crusaders this year. He did some good work at the breakdown and on defence, though he was bumped off by a charging Taulupe Faletau that night - part of a pack well overshadowed by the tourists.

Dixon has played three tests (all as a sub) and did well enough - but an ordinary 2017 saw him slip down the rankings. His Super Rugby campaign in 2018 will be vital.

Ardie Savea has been tried at No8 and certainly has the speed and ball skills for the position. But he, too, is a smaller loose forward; many memories may roll back to 2003 when the All Blacks lost to England in Wellington and another well-performed but smallish No8, Rodney So'oialo, was rag-dolled by bigger English forwards.

So it may be Squire to No8 and Fifita given an extended run at 6. If it's youth you are looking for, there will be much interest in the Super Rugby debuts of the Chiefs' Luke Jacobson (skipper of the world champion New Zealand under-20s side) and Ethan Blackadder in the Crusaders (son of Todd).

But, for a real roughie, what about Taleni Seu of the Chiefs? This raw-boned loose forward-lock is an athlete who stands more than 2m, has an insistent, bruising style and a frame that adapts easily to lock - the sort of combination that could win a spot in another depth-building end of year tour in 2018.

Particularly if Kieran Read doesn't make it. Or even if he does.