Hager was sympathetic about players needing a break but reminded each one they didn't have a right to walk back into the side on their return.
"One of the things I say to them is sometimes you have to be very wary about taking time away from the game because you lend your stick to other players," Hager said. "There's always that risk that others improve so much they take your position.
"By all means, I am supportive of it but I also make it quite clear there are no guarantees of future selection. It comes down to performance."
Harrison will play for Antwerp side Braxgata in the Dames 1 league, the team she played for in 2008, but will base herself in Rotterdam with partner and men's international Steve Edwards, who plays in the Dutch league. Antwerp is about one hours' drive away.
"The level of hockey and the pressure of it is a lot less than international hockey so it will be a break but I will still be keeping up with my stick skills," Harrison said.
"There have been several players who have had some time out. I went back for the India series [last month] and I felt like I wasn't quite ready. Having an extra few months off will be enough for me to come back raring to go. It's a really good experience over there. It's quite relaxed, which is what I need right now. It's worked out perfectly."
It weakens a Black Sticks side already facing a stern test. Not only will they take on the world's No 2-ranked side on their own turf - New Zealand are ranked third - but they also arrive in Argentina only one day before the first test because of difficulties around flights.
Hager has included eight players from the Olympic squad along with nine others from the Indian series. One final player will be added once the squad returns to training next week.
"It's going to be very difficult, we are not going to hide behind that," Hager said. "We are going to be challenged and questioned on a lot of things, particularly our mental side. We are there for only 11 days and we will just be starting to get over jetlag by the time we come home.
"I couldn't ask for a better environment to throw these girls into and find out who can sink and who will swim and who has the mental strength to push all those barriers to one side and play consistently well."
New Zealand squad to play Argentina in a five-test series starting February 24: Bridget Blackwood (Canterbury), Sam Charlton (Midlands), Gemma Flynn (Midlands), Krystal Forgesson (Auckland), Sian Fremaux (Canterbury), Amelia Gibson (Canterbury), Katie Glynn (Auckland), Rose Keddell (Midlands), Pippa Hayward (Canterbury), Olivia Merry (Canterbury), Stacey Michelsen (Northland), Emily Naylor (Central), Anita Punt (Capital), Cassandra Reid (Canterbury), Aniwaka Roberts (Capital), Bianca Russell (North Harbour), Petrea Webster (North Harbour).
