The 32-year-old was an unused substitute in the 1-0 Europa League last-32 first leg win over Sparta Prague on Thursday, despite being available again following a setback in his recovery from a knee injury.
Terry has made two starts and one substitute appearance under Benitez, having been injured at Liverpool prior to the Spaniard's appointment, and the interim boss will only select the former England captain if he can train fully.
"He has to train normally three, four training sessions to be ready for the games," said Benitez, who has named Terry on the bench without using him five times.
"Then if the others are playing well he has to compete (for selection)."
Gary Cahill and Branislav Ivanovic were deployed in central defence in Prague, with Terry warming up frequently on a sub-zero evening in the Czech capital.
A fierce competitor such as Terry would be keen to play in any game, but managing his fitness and not his character is Benitez's concern.
"The main thing is the knee," Benitez said.
"If he's fine, after we have to manage the other things. The main thing is if he can train every day he can be ready for every game. After we will see.
"The good thing is we have now Cahill and Ivanovic playing well and also David Luiz as an option.
"We now have four options at centre back. We have Terry, David, any of them they can complement (each other) really well.
"If everything is fine, they have to compete. If something is wrong we have different options.
"All the players, they know, that at this level when you play so many games you have to train and you have to keep fit.
"It's not a question of 'no, I can carry on and play' and after not train. Everybody knows they have to compete and everybody knows we have good players now."
The Brentford clash will be FA Cup holders Chelsea's 45th game of the season, meaning squad rotation is a must.
As well as Terry and Ashley Cole, who was also an unused substitute on Thursday night, Luiz, Demba Ba, Victor Moses and John Obi Mikel are poised to return.
Griffin Park at the end of January might have come as a shock to some of Chelsea's most cultured players, with a 2-2 draw the result, but Benitez believes it gave them a flavour of the uniqueness of the FA Cup.
Benitez, who lifted the trophy with Liverpool in 2006, said: "The atmosphere was quite good for the local team. I'm sure they (the Chelsea players) were impressed and hopefully they will be ready when we play at home and the atmosphere has to be good for us.
"Because we have seen them we know now what we can expect. I think that it will not be a surprise for anyone.
"In a cup competition that you know you cannot make mistakes. It's important to be sure that you approach the game without any complacency.
"From the first minute you have to concentrate and do the right things."