A five-game unbeaten run to end 2011 helped move Sociedad out of the relegation zone but they remain just two points above the drop zone in 15th.
Montanier told the club's website: "The league and the cup are two different competitions and the present is more important than the past or future.
"We obviously want to extend our good run, even more so because it is a derby match. However Osasuna are picking up a lot of points, they concede very few goals and are dangerous on set-pieces and the counter attack. It will be a very difficult match."
Inigo Martinez is suspended for the hosts after his red card against Racing Santander three weeks ago and Asier Illarramendi remains out injured.
Osasuna are looking to keep their own two-month unbeaten league run going in San Sebastian.
However coach Jose Luis Mendilibar has again tried to dampen expectations of Osasuna challenging for a European place despite currently sitting in fifth position.
"The objective for 2012 is to stay in the league," he told the club's website.
"What is important right now is the match against Real Sociedad. They are a direct rival in all aspects and they are on a high right now. They now have the confidence they were lacking before."
Mendilibar has a number of injury concerns with Raul Garcia, Roversio Rodrigues and Ruben Gonzales all doubtful. However Ibrahima Balde has not been selected by Senegal for the African Nations Cup and will therefore retain his place up front.