The Portuguese striker, signed from Wolves, has scored nine goals in 15 outings for the Reds.
"No one could've predicted that he would hit the ground running like he has," Owen told BT Sport. "You'd have to say he's the best signing of the season so far in the Premier League.
"He's surprised me with his pace, I must admit. I think he's far quicker than I gave him credit for. Against Atalanta he was electric.
"He just shows composure, a lot of people panic, they just get a shot off as soon as they get any form of space.
"I just think he's scored headers, right foot, left foot, he's scored everything so far and he's looked the real deal in terms of linking play.
"Nobody could have envisaged the quality that he's shown so far."