The former Manchester United striker tore knee ligaments in Hannover's 2-0 defeat to Wolfsburg in April and underwent surgery soon afterwards.
His recovery is going according to plan and he has now been given the green light to start light work with his team-mates.
"Our medical department examined
"But he still needs time to make a full recovery. We cannot determine when he will be able to play again at the moment."
Borussia Dortmund defender Mats Hummels insists the UEFA Champions League final will not be his last game for the club.