The 44-year-old has missed the last three months of the season after suffering a relapse of the cancer he was treated for in late 2011.
However, after successfully undergoing chemo and radiotherapy treatments, Vilanova could return to the sidelines for the latter stages of this season.
"There is no doubt that a group gets weaker when they lose their leader," Rosell told ESPN. "Sports psychologists say, and I suppose there is a scientific explanation to it, that after 30 days the group's shape weakens.
"We have been without our leader for a long time now and it is normal to see this happening. He is longing to come back, hoping for the process to finish, which is due at the end of March.
"The other day he told me he was happy because he could start the countdown as he is over half of the process and he has less than 50 percent of it left. That is very good for his state of mind. He wants to come back and we want him back."
Assistant manager Jordi Roura is currently taking charge of the club.