Griezmann hit the post and had another chance of scoring during the 2-0 Champions League win.
After using the France attacker sparingly so far this season, Koeman offered Griezmann encouraging words, stating: "We had a very good chance to score more goals.
"Antoine (Griezmann) was unlucky tonight. His first strike early in the match, he could not do better and the same for the second.
"He has to keep working, but he's getting opportunities, which is positive."