Shoemark and another of Horrell's Carters Frame and Truss Taradale teammates, Billy Ropiha, are expected to battle for his No12 jersey in his absence.
"They are both good guys and whoever gets the job will play to the best of their ability and do the region proud," Horrell said.
Many Magpies fans will struggle to believe Horrell has been hindered by the injury on the left side of his hip he collected during the Super Rugby season. As he has throughout his 42-game career with the Magpies and 35 with the Chiefs, Horrell, 26, has again been the "Mr Reliable" in the Magpies' midfield this season.
"After the surgery, I will have a bit more flexibility back and will be ready to go come the start of the Super season," Horrell said.
"I'll be 100 per cent when I get back to it and, hopefully, able to get more game time. At the same time, there's always competition for the Chiefs midfield berths so when you get the opportunity you have to make the most of it," Horrell replied when Sonny Bill Williams' return to the Chiefs was mentioned.
The top tackler among the backs with nine during last week's loss to Northland, Horrell, said he believed the mental side of the Magpies game required the most attention after the trip to Whangarei.
"We let the foot off the throat and you can't afford to do that against any team, especially Northland in Whangarei. Otago will be equally as hungry after we took the shield off them last year."
The top points scorer in the 2011 ITM Cup with 152, Horrell pointed out the Magpies have placed a major emphasis on shutting down the Razorbacks' space tomorrow.
"Otago play with a lot of movement. It's crucial we shut down their space, allow our big boys to dominant up front and this will allow us backs to play around," Horrell said.
He admitted to knowing a bit about Razorbacks centre Kieran Moffat but nothing about their second five-eighth Tei Walden.
"It's important we play with attacking flair, how we always play and not let the occasion get to us," Horrell said.
The Christchurch Boys' High School product got to know his Magpies midfield partner Robbie Fruean at the Chiefs and has enjoyed playing with him at ITM Cup level. Their defensive work in the midfield will have to be first class if the shield is going to stay put.
The Magpies must be careful not to concede too many penalties in their own half. The goalkicking feats of Razorbacks first five-eighth Hayden Parker have been well documented in recent seasons.
In fact, if the game turns out to be as close as it has the potential to be it could be decided by the respective goalkickers, Parker and Magpies pivot Ihaia West. It was pleasing to see West having a couple of dropped goal attempts at training yesterday as one of those could also prove the difference.
Quality defensive work from the hosts handy to the fringes will also have to be at its best also if the likes of Razorbacks flanker TJ Ioane and captain and No8 Charlie O'Connell are going to have their freedom restricted. Give them too much room and the shield could be gone.
Lock Tom Franklin and hooker Liam Coltman are other handy players in the visiting pack. Coltman's tussle with Magpies vice-captain Ash Dixon will be an early Highlanders trial for the pair.
By 6.10pm yesterday, 9200 tickets had been sold for the game.
Hawke's Bay Rugby Union commercial manager Jay Campbell said with a capacity of 18,500 there were still plenty of seats left, the majority being in the Ticket Direct stand which are selling for $12.50 and $7.50 each.
