Napoli lost 3-0 on Thursday night.
"We felt a little ashamed," wrote Hamsik on his official website.
"It went badly and our fans did not deserve that. We had the chance to equalise at the start of the second half and I could've scored for 1-1, but it didn't go the way we wanted.
"Now we must immediately look forward to the game with Sampdoria, because the Viktoria match is finished and there's nothing we can do about that now."