The Swede showed a test of his speed and pulled away from three Eibar defenders. Once inside the penalty area, he shot hard and well placed 1-0 with the left.
In the past, 19-year-old Isaac has scored three goals in four training games for his new team.
In the match, however, there were no more goals for Isak - or his teammates. At the same time, Eibar found it right twice and turned and won by 2-1.
The training match was Real Sociedad's last rally for Saturday's league premiership away against league fixture Valencia.