Terry has been unable to play consistently for Chelsea but Benitez believes he made a step forward over the last week.
"He was training with us this week and he was doing well. He is training every day and improving his fitness," Benitez said.
"It is just a question of how many games can you play? We will have four games in nine days. It is the same for everyone.
"You can't play all these games and be 100 per cent - not just him, everyone.
"We have to ensure we have a competitive team against Southampton and a competitive team against United. We have to manage the squad carefully.
"The most difficult one will be these two games (coming up) because they are so close."