The hosts controlled a first period that was disturbed a fierce wind in Pamplona and could easily have been in front before the break as Armenteros saw a header come back off the crossbar and Alvaro Cejudo was twice denied by good saves from Javi Varas.
However, the goal did arrive just 10 minutes after the break as Lolo cushioned a header into the Argentine's path and he rifled the ball low past Varas to register just his third goal and Osasuna's fourth home league win of the season.
Celta were missing two of their most creative outlets in Augusto Fernandez and Michael Krohn-Dehli through suspension and their presence was sorely missed throughout as the visitors struggled to carve out openings.
Armenteros was first to threaten as Osasuna started brightly but he could not direct his header from Cejudo's cross on target on 22 minutes.
Moments later the same two players combined again but although the former Rayo Vallecano man got a better contact on his headed effort this time, the ball came back off the crossbar and into Varas' grateful arms.
Borja Oubina then had a rare chance for Celta as he tried to catch out Andres Fernandez after the goalkeeper sprang from his line to prevent Enrique de Lucas having a clear run on goal but the midfielder's effort was wildly off target.
The visitors were soon on the back foot again as Varas had to make two smart saves in as many minutes from Cejudo to keep the scores level at the break.
Nino also came close after the break as Osasuna again started the half on top and their persistence eventually paid dividends when Lolo intelligently headed a corner into Armenteros' path and this time there was nothing Varas could do to prevent the ball flying into the bottom left-hand corner.
Thereafter Osasuna retreated slightly, comfortable with holding onto what they had, and apart from one Alex Lopez cross which both Park Chu-Young and Vadim Demidov just failed to get on the end of 11 minutes from time the hosts held on comfortably for a vital three points which move them up to 16th.