The Brazilian was forced off during the 2-1 victory over Blackburn on Sunday with a problem in his right leg.
The 30-year-old missed the start of the season after suffering a freak knee knock in the summer.
Benitez said: “It’s not good news and we think he is going to be out for maybe another two or three weeks.
“Fabio has had really bad luck.
“It all began at the start of the season, when he suffered the injury during the summer with his family, and it’s like there has been something every month since then.
“It is a shame because he is a top footballer and a really nice person but him being out leaves us with a problem because we wanted to play him, so we could give Emiliano Insua a rest.
“We needed Fabio so he could help the team and also help Insua. His injury is bad news for both of them.”
