I'm not sure anyone's ever said or implied that. It doesn't make sense.
Normally he would be a 'serious' investment. If I recall, last year Deportivo wouldn't let him go for less than 20 million euros. It's only because of the relegation clause that we were able to get him so cheap.
I think...
A poor goal scoring record?
Averages a goal every 253 minutes over the last 3 seasons in La Liga. That's better than a goal 1 every 3 full games. If he reproduces that form in the Premier League he could score around 12-15 goals next season.
A top class striker would average a goal every 2...