The Spaniard spent just £1.8million – the fee paid to Bayer Leverkusen for Demarai Gray – in his first window, with four other players arriving on free transfers.
Benitez, who as a former Liverpool manager was controversially appointed in June, hoped to have been able to do more business but that was reliant on offloading players, which never happened.
He said: “We didn’t spend a lot of money, but we had good signings. It could be better, yes, because if you have more money you can do it, but the financial fair play rules were killing us in this window.
“I was not expecting (this) at the beginning because I was thinking maybe we could sell some players and then we could do well in the window.
“Hopefully in the next window it will be different, but the players we brought in were players that we needed.
“Maybe we needed players in more positions, but it’s what we could do and I think we did quite well.
“When I say ‘next window’, hopefully it will be January if it’s necessary. If we are doing so well that in January we don’t need anybody, I think everybody will be happy here.
“We have to start working now, thinking about how to improve the squad, first of all on the pitch, and then afterwards bring in players if we can.”
A key area he could improve his squad in relates to James Rodriguez, one of the players he wanted to sell in the summer but could not because of the player’s high wages.
The club would still be open to offloading the Colombia international, who has yet to feature in a matchday squad, but Benitez set the former Real Madrid playmaker a challenge.
“He was available in the transfer window. Now that we have just the Middle East window open, it’s not easy,” added the Everton manager.
“I think he realised he has to improve in a lot of things. He has to focus, he has to show his commitment. That’s what he’s trying to do and that is good for us.
“If he stays with us, it’s good news for us because he can give us something more. We don’t have any doubt about his quality, we have to be sure that we have players playing for 90 minutes with the intensity we’re looking for.
“If James is staying with us, we need to find the right games for him, and he has to show his commitment and his desire. It’s something his team-mates, fans and staff are expecting for him.”