The 27-year-old only completed his move from Newcastle on Friday and Chelsea interim manager Benitez was pleased with what he saw.
"The first half we had to work but after that little by little the team was getting better," he said. "The second half was much better. We played with quality and when you're one goal ahead they have to take more risks.
"It is always important for a striker to score goals but more than just the goals it is the contribution for the team.
"The understanding of things we were talking about like how to know the movements of other players, he was very clever and could understand everything."
Ba's display was so impressive that Fernando Torres, a player rejuvenated under Benitez, was left on the bench at St Mary's.
"I prefer this problem than just to have just one," the Chelsea boss said when asked if the pair will prove a selection headache.
"It is important for the team to have these different options, good players. They can compete or they can play together, depending on the game.
"They can play together I think. He was playing at Newcastle with Papiss Cisse and can do it because he is a clever player."