"He will have to undergo examinations tomorrow or the day after," Blanc told a news conference. "It's obviously a muscular problem so I think we have lost him for a little while."
Thiago Silva appeared to receive the injury in the 15th minute by jumping for a defensive header and immediately asked to be replaced.
With Alex already out of the game injured, the French champions had to play with an unusual pairing of close season recruit Marquinhos and substitute Zoumana Camara.
PSG, who had taken an early lead thanks to Zlatan Ibrahimovic, looked disorganised when Silva walked out of the pitch and conceded the equaliser to Radamel Falcao in the 20th.
"You have to be realistic. You can't lose Thiago Silva without experiencing some uncertainty," Blanc said.
"We all know what a player he is and we know that he's very reassuring for his team mates."
Thiago Silva could miss at least two league games, with the next round of matches taking place on Wednesday, as well as next week's Champions League home games against Benfica.
Both PSG and Benfica have won their opening game, 4-1 at Olympiakos and 2-0 against Anderlecht respectively, and are expected to compete for the top spot of their group.
Reporting by Gregory Blachier; Editing by John Mehaffey