Hertl did not take part in the Sharks prospect scrimmage Wednesday because of the knee, which sustained torn ligaments in December.
"My last time skating was the World Championships (in May)," Hertl told the newspaper. "And here, first time, it's just a little bit sore."
Hertl's rookie season was limited after he tore ligaments in his right knee Dec. 19. He returned to play two regular-season games and seven against the Los Angeles Kings in the Western Conference First Round.
Hertl, 20, had 15 goals and 10 assists in 37 regular-season games, including four goals against the New York Rangers on Oct. 8, his third NHL game. He had two goals and three assists in seven Stanley Cup Playoff games.
Hertl practiced during the first two days of development camp this week, the report said.
"Stronger in whole body and stronger [in] my knee," Hertl said.