Jay Rodriguez gave Saints a 22nd-minute lead in the FA Cup third round tie but Ba, who completed his £7million move from Newcastle on Friday, levelled 13 minutes later and he then added a second after the break.
Victor Moses, Branislav Ivanovic and Frank Lampard were also on target for the holders.
Benitez told the club's official website: "It is always important for a striker to score goals but more than just the goals was his contribution for the team.
"His movement and understanding of things we were wanting to do was really good. He was very clever and he could understand the movement of the players. He did everything well on the pitch.
"I prefer to have this selection problem now than to have just one striker. It is important to have good players and different options, and Ba and (Fernando) Torres can compete or play together."
On his side's overall performance, the Spaniard added: "The first half Southampton started well but little by little we were doing better, and the second half we were much better.
"With the players we have, when you go one goal ahead and the opposition have to take more risks, if you have clever movement you know you will score more goals."