Jos Buttler has emerged as a major doubt on the eve of England's second Test against New Zealand at Hamilton after the wicketkeeper-batsman tweaked his back during a gym session.

Buttler had only returned behind the stumps in the series opener at Mount Maunganui, where England's innings-and-65-run defeat left New Zealand in an unassailable 1-0 lead in the two-Test series.

If Buttler is ruled out then Ollie Pope – who has kept in only five of his 33 first-class matches and most recently for England Lions against India A in February – is primed to take on the gloves at Seddon Park.

England captain Joe Root said: "Jos has tweaked his back in the gym, we'll have to find out a little bit more information on that throughout (the next 24 hours) and see where he's at.

"We knew that this was a possibility when we selected the squad and I'm quite happy that Popey's got the capability of doing a good job for us."

Pope was going through some wicketkeeping drills on Thursday afternoon in England's optional training session, where only Buttler and paceman Jofra Archer did not make an appearance.

Buttler's potential unavailability means England could hand a Test debut to Zak Crawley, the spare batsman on tour, although Root hinted they are considering recalling seam bowling all-rounder Chris Woakes.

Root added: "There's a number of different combinations we could go with and I think a lot of that will rely on the surface.

"Trying to find the best combination which would take 20 wickets and then balancing that in terms of getting the batting where we want it to be. It could be an all-rounder, we'll see."

Buttler had assumed the wicketkeeping role from Jonny Bairstow following the conclusion of the Ashes, with the Yorkshireman kept on standby after the Twenty20 series following an injury to Joe Denly before returning home.

Bairstow was left out of the squad for the ongoing series after averaging a paltry 20.25 in nine Tests this year – although he was central to England's historic World Cup win earlier this summer.

An average that was once well in excess of 40 has now dropped to 35.26 in 69 Tests, a figure Root believes is significantly less than what someone with Bairstow's talent is capable of achieving.

While Root was cautious about confirming whether Bairstow's best route back into the team was as a specialist batsman, he would back his Yorkshire teammate to make notable contributions if and when he is recalled.

Root said: "You can't rule out anything out.

"For Jonny you look at his talent and what he's achieved in Test cricket and what he potentially could achieve in Test cricket and you probably think he's got the capability to go on and make really big scores and be one of the best players in the world as a batter alone.

"For him, he's just got to continue to keep looking to improve and work hard at his game. When he gets his opportunity wherever that is, take it. He's someone that's very good at doing that when that happens."

Bairstow will be the lone batsman on a pace bowling camp going out next month to South Africa alongside Mark Wood and James Anderson, with the trio hopeful of pressing their claims for the Test series against the Proteas.

Asked whether Bairstow is in line for a squad place in South Africa, Root responded: "No. It just means he's got an opportunity to push his case and that selection will happen when we get back.