So it means either Patel, who took 13 wickets against Pakistan after making his debut in the first test, or Somerville, who finished with match figures of 7-127 in the third test, will miss out on a first test at home.
Or possibly both. Mitchell Santner and Todd Astle are injured but Ish Sodhi remains the most experienced of the Black Caps spinners with 17 tests.
Sodhi played one test at home last summer, against England at Christchurch, where he was the hero with the bat, saving a test draw and series win. However, he failed to take a wicket in that test and through five tests at home has an average of 93.80.
Patel might have the edge considering he has been the top wicket-taker in first class cricket for the past three seasons and was named domestic men's Player of the Year last summer after helping Central Districts clinch the Plunket Shield.
The New Zealand squad for the first test is named today.
Black Caps coach Gary Stead had plenty of praise for the spin-bowling debutants yesterday.
"For both to have highlights as they did early in their careers is a really big coup for them," Stead told Newstalk ZB's Martin Devlin.
"They always say you become an elite cricketer in your late 20s or early 30s before you really understand yourself.
"While they haven't had a lot of international experience, what they do have is a lot of cricket under their belt, and I guess a lot of miles there as well. They have played a lot of cricket and have confidence in their abilities."
After three spin-friendly pitches in the UAE, Black Caps pace bowler Tim Southee said yesterday he was hoping for a little more assistance at the Basin Reserve.
"We're looking forward to Wellington. Hopefully there's some grass on the wicket. And I'm sure the bowlers will be pretty excited in the Basin Reserve nets in a couple of days."