The players cost the Portuguese club £5m each and their return will be invaluable to a side that has struggled with injuries all season.
The Argentinian duo both pulled out of their country's squad last week for the friendly against Saudi Arabia in order to be ready to face Celtic this week. Benfica are in desperate need of three points in Group G, with their final group match presenting an away tie at Barcelona.
Garay, signed from Real Madrid, has recovered from a thigh strain while Perez has overcome a hamstring knock. Benfica will find out later today if Nemanja Matic will be fit for the match with Celtic after the midfielder picked up an injury in Benfica's 2-0 Portuguese Cup win over Moreirense on Friday.
"We have a problem if Matic is injured," said Jorge Jesus, the Benfica manager. "He scored a goal for us, but even before the goal he had been the best player on the pitch. He is a serious doubt for the game against Celtic."
Aside from injuries, Benfica have also welcomed back club captain Luisao from a two-month ban he received during a pre-season match with Fortuna Dusseldorf. The Brazilian was punished for pushing German referee Christian Fischer to the ground.
He made his return against Moreirense and his presence at the back will be a real asset against Celtic. "He is a very important player for us in the dressing room and on the pitch," said 20-year-old defender Luis Martins.
"To have him back in the side for the Celtic game is vital to us. He is a captain, a leader, and a player we look up to. "