The forward grabbed a brace in his third appearance for the Great Danes after replacing injured Loic Remy to ensure his side clinched their second win of the season.
Galtier has praised the man-of-the-match display from the striker but admitted the forward still needs to improve on his performances.
"Victor Osimhen is only 20-years-old, so he has a lot of room for improvement,” Galtier told the club website.
“He's going to get better physically because I remember he only had seven days off this summer. I hope that he will go better and better.”
The 20-year-old teamed up with Lille in the summer as a direct replacement for Nicolas Pepe, who joined Premier League side Arsenal in a club-record fee of £72 million.
Osimhen immediately hit the ground running, scoring a brace in his debut appearance for the Great Danes in their 2-1 victory over Nantes.
Despite the convincing showings, the forward who is part of the Super Eagles 24-man squad for September’s friendly game against Ukraine, was dropped to the bench with Remy preferred ahead of him.
“I had made the choice not to start him, despite his magnificent start especially against Nantes, because after the match in Reims, Victor will fly to participate in two games with his national team,” the Lille boss continued.
“I was very happy with Loic Remy during the preparation, he unfortunately quickly hurt himself. This is the advantage of having a workforce like ours, with Victor, who came back without asking any questions.
“He understood my choice to put him on the substitutes' bench, and he came back to be decisive. I think about the goals, of course, but also about what I'm looking for in the game of an attacker: to put intensity and to press forward."
Osimhen will hope to maintain his form when Lille take on Reims in their next league game on Sunday.