Lukaku has played just 64 minutes of football this season.
The ex-Inter Milan forward has 124 caps for his country and is aiming to feature at a fourth World Cup this summer.
Belgium’s all-time leading scorer, with 89 goals, said on Monday that he “could never turn his back on Napoli” and that he wants nothing more than to “play and make my team win”.
Lukaku, whose father Roger died in September, said: “This season has been very challenging for me, between the injury and personal loss.
“I know there has been a lot of noise over the last few days about my situation and it is important to clarify everything.
“The truth is that in recent weeks I didn’t feel well physically, I underwent some checks while I was in Belgium and it turned out there was inflammation and fluid in the iliopsoas muscle, near the scar tissue.
“It is the second problem I have had since I returned at the beginning of November. I chose to do the rehabilitation in Belgium so that I can be ready when called upon.”
