Hamsik exacerbated the existing issue after coming on as a second-half substitute in the 1-0 Serie A defeat to Parma on Saturday and was subsequently ruled out of this week's UEFA Champions League clash in Germany.
Scans revealed that the 26-year-old had avoided a fracture but Hamsik - who first sustained the blow in a 3-0 loss to Juventus, two weeks prior to the Parma game - did not get off as lightly as he had previously thought.
"I already had some pain after the Juventus game," he told Sportky.
"It's an inflammation to my fourth toe on my right foot. I didn't think it was that serious, and before the Parma game I trained on my own throughout the week.
"But unfortunately, I was forced to leave after just a few minutes (after coming on against Parma). We'll have to see how it is."
Hamsik has scored six goals in all competitions for Napoli this season, but Rafael Benitez's men have lost three matches in a row after the 3-1 reverse against Dortmund.
