The Brazilian has made two high-profile errors in the last month, giving away a late penalty in the draw at Wigan and then getting sent-off in the midweek FA Cup clash at Everton.
Some Liverpool supporters have been venting their frustration at the 22-year-old, who will be suspended for Saturday evening's Premier League clash at Portsmouth.
But Benitez says the midfielder has his full backing and the manager remains keen to point out that Lucas still has a big part in his first-team plans.
The Spaniard told the club's website: "We have some problems and he is a player who could have been really important for us.
"I can't speak for every single fan, but every member of staff knows he is a very good player and his teammates know he is a good player.
"Against Wigan, for me, Lucas was the best player on the pitch. Then the other day at Everton he was also playing well and the ref made three mistakes."
Benitez added: "I will support Lucas 100 per cent, always. He is a fantastic player, he is a nice boy and a very good professional. He has quality, he is learning and is improving all the time.
"It is not difficult for me to say that I support him 100 per cent."