The striker, signed on a season-long deal from Shakhtar Donetsk, passed a medical ... but is yet to kick a ball for the club.
Ferreyra says he won’t be involved against Southampton at the St Mary’s Stadium tomorrow, because he has "a back problem".
Alan Pardew must now be thinking seriously about whether he was wise to sign the player, who has a history of "mystery" injuries.
Alan Pardew: “Unfortunately, he’s hurt his back.
“We’ve had a bit of a testing couple of weeks really in terms of injuries. That’s a shame, but it’s not a long-term one.”