Huntelaar was substituted in the 33rd minute after a tackle by Real defender Raphael Varane, and initial reports on social media claimed the Dutch forward had broken the fibula in his right leg.
But after the UEFA Champions League round of 16 first leg, which ended 2-0 in favour of Real Madrid, Schalke revealed Huntelaar had watched the end of the match at the Veltins Arena, describing his injury as a "large bruise".
"Huntelaar suffered a large bruise tonight. He was back in the Veltins Arena by the final whistle though," Schalke wrote on their official English Twitter account.
A more specific diagnosis is expected later in the week after Huntelaar is examined at a hospital.
Di Matteo reiterated that Huntelaar had not broken his leg, although Schalke's coach was unsure when the 31-year-old might be available to play again.
"Nothing's broken [but it is] very painful. We can't say how long he'll be ruled out," the former Chelsea and West Brom manager said at his post-match media conference.
