The six-foot six, three-times All-Star has already been ruled out of the first round of the playoffs against the Phoenix Suns but he could return should Portland advance.
The 25-year-old Roy, who leads Portland with 21.5 points per game this season, was injured in Sunday's 91-88 win against the NBA champion Los Angeles Lakers.
"We know that's going to be a challenge," McMillan said of Roy's absence from the best-of-seven first round of the playoffs which starts on Sunday in Phoenix.
"Regardless of what we've done in the past without Brandon, it's a new season. We're going to have to do it again and have ourselves ready to go."
(Writing by Mark Lamport-Stokes; Editing by Alastair Himmer)