Lampard, a 65th-minute substitute at St Mary's, rounded off the scoring for the FA Cup holders with a late penalty.
It saw him equal Kerry Dixon's goal tally of 193 for Chelsea to put him joint second behind Bobby Tambling, whose record haul of 25 FA Cup goals Lampard drew level with in the process.
"It is a great achievement for him and hopefully he can score a lot of goals until the end of the season," Benitez said. "We will be happy and he will be happy if that happens."
Lampard's current deal runs out at the end of the season and asked whether there had been conversations over an extension, Benitez said: "As I said before, Frank is a good professional.
"He is doing well and is under contract and he is fine. I know they have some conversations sometimes, but the main thing for me as a coach is to get the best out of him and that's it."