The Anfield boss surprised almost everyone in the stadium by consigning Gerrard to the bench and only bringing him into the 2-1 win over Real Betis in the final 19 minutes of a tense match.
Gerrard has played two big internationals for England against Wales and Northern Ireland in the last fortnight, as well as Saturday's league game at Spurs.
Benitez said: "We tried to protect Steven, there were a few players in the same condition. I felt we needed fresh legs for a very difficult match.
"Some people were tired, including Steven, and we needed to rest players for this match."
Benitez also left Steve Finnan, Stephen Warnock and Djibril Cisse on the bench, all of whom had been involved in international squads of late.
Liverpool went on to produce a display of attacking excellence and then sheer defensive defiance to claim what could be a priceless victory.
Two goals in the first 14 minutes from Florent Sinama-Pongolle and Luis Garcia gave the holders a foothold on the match they should not have conceded.
After dominating the first period they found themselves under siege from a Spanish side making their debut in the competition.
Arzu pulled one back early in the second half and from then on it was a backs against the wall display which proved enough to give Liverpool the critical start they needed as they began the defence of their title in earnest.
Crucially in the intense heat, Liverpool started with players who knew the conditions; Djimi Traore and Josemi were the full backs, Sissoko - at Valencia last term - took Gerrard's midfield role and Xabi Alonso returned while French youngster Sinama-Pongolle was with Peter Crouch up front.
Betis had Sanchez Joaquin recovered from a knee problem but were without Luis Fernandez.
The Estadio Manuel Ruiz de Lopera was rocking with noise but the home fans were stunned within 90 seconds when Liverpool took the lead.
Carragher's long ball up to Crouch and Sinama-Pongolle sparked instant fear in the Betis defence and young Sinama-Pongolle took the ball out of the air with excellent skill.
It clipped Crouch inadvertently, and fell for Sinama-Pongolle to curl a delightful chip over a stranded Antonio Doblas.
Liverpool struck again on 14 minutes, and Crouch - being watched by England boss Sven-Goran Eriksson - played a major part.
He controlled and kept possession on the left, and searched for a telling target, eventually pushing the ball cleverly into Bolo Zenden's path.
The Dutchman's instant low cross was met by Luis Garcia arriving unmarked in the box and he clinically slipped the ball into the corner of the net past Doblas' despairing right arm.
Betis were stunned but slowly regained composure and Brazilian dangerman Ricardo Oliveira gave Sami Hyypia the slip before surging into the box to force an excellent block from Jose Reina.
Reina was Liverpool's saviour again when a corner dropped into the six yard box and Oliveira stabbed at it, but the Spanish international fell on the ball right on the line, and he saved well again when he blocked an angled drive from Fernando.
Betis took off defender Gutierrez Juanito at the break and sent on wide man Munoz Xisco in an attempt to stretch Liverpool's defence.
Carragher was booked for a foul on the nippy Dani, and Betis' heavy early pressure in the second half produced a goal on 51 minutes.
Dani's ball evaded Hyypia and Arzu was clean through the Liverpool defence to easily beat Reina.
Liverpool brought on Cisse for Crouch, then John Arne Riise for Zenden before Gerrard took over from Sinama-Pongolle, but the relentless pressure continued from the Spanish side.
Reina - who was booked in the final seconds for kicking the ball away from a corner kick - again came to Liverpool's rescue when he plucked down a curling Marcos Assuncao free-kick after a foul by Traore on the edge of the box.
: Doblas, Melli, Juanito (Xisco 45), Rivas, Oscar Lopez, Arzu (Capi 72), Fernando (Dani 35), Assuncao, Joaquin, Varela, Oliveira.
: Reina, Josemi, Carragher, Hyypia, Traore, Luis Garcia, Sissoko, Alonso, Zenden (Riise 66), Sinama Pongolle (Gerrard 74), Crouch (Cisse 58). Referee: Konrad Plautz (Austria).