
The Spain forward returned from two weeks out to face the Gunners but was clearly unfit and was taken off at half-time.
"A scan this morning has shown a tear to a different adductor muscle to his original injury and we expect Fernando to be out for about three weeks," a Liverpool spokesman said on the club's website.
"As for (midfielder) Xabi Alonso, an X-ray showed a fracture to the fourth metatarsal in his left foot. Xabi will be seen by a consultant tomorrow after which we will have a much better idea of the timescales involved."
Liverpool are still unbeaten in the league this season but lie sixth in the table, six points behind Arsenal and Manchester United.
DPA