The 24-year-old was placed on his side's injured reserve list after injuring his knee in the Packers' pre-season clash with the Seattle Seahawks.
"It's a tough one," McCarthy told reporters. "He's definitely a young player I felt was going to be an impactful player on offense.
"I know it's a tough blow for him. We need to get him healthy and we will start that process."
Green Bay open their regular season campaign against the San Francisco 49ers on September 8.