The Chicago Daily Herald first reported the story Tuesday night.
A Blackhawks spokesman told ESPNChicago.com Wednesday morning that the team was aware of the injury before signing Hossa, who was an unrestricted free agent, to a 12-year, $62.8 million deal on July 1.
The team spokesman said the injury happened sometime during the playoffs, when Hossa was playing for the Detroit Red Wings.
Hossa, 30, is rehabilitating the shoulder, and the Blackhawks spokesman said it has not been determined if surgery is needed, and that the decision is up to Hossa and a doctor.
According to the Daily Herald, the injury might be related to the rotator cuff, but the severity is not known.
Hossa had 40 goals and 31 assists in his sole season with the Red Wings, who defeated the Blackhawks in the Western Conference finals before losing to the Pittsburgh Penguins -- Hossa's former team -- in the Stanley Cup finals.