Rohan has been battling several niggling injuries during the pre-season, most notably a back complaint and will miss the opening three rounds of the regular season.
Meanwhile, Johnson, who has undergone five knee reconstructions in the past five years, will also not be available until round four.
“Gary has been limited in his training, most recently because of back soreness,” head of football Tom Harley said.
“As a result of his limited training he will need at least another eight weeks of preparation before he can return to play.
“Gary had a similar start to last season and was still able to come in a make a significant impact, so we’re confident he will do the same again this year.
“We’re really pleased with where Alex is at in terms of his rehabilitation, he’s been gradually increasing his training and is in great spirits,” Harley added.
“By placing Alex on the long-term injury list it will enable him to be available for selection from round four, and we’re really hopeful of seeing him return to playing this year.”