Lukaku, having grabbed his sixth goal in three games for club and country, was forced off after 66 minutes and replaced by debutant Enner Valencia.
And though Koeman had a positive update on the Belgian, he admitted he could now be a doubt for Tuesday's League Cup visit of Norwich City.
“He's okay, he had a knock on his feet," he said. "We have to wait. Tomorrow we are in and then we will know how the reaction is and if he is available or not for Tuesday."