Dublin hasn't been impressed by the Spain international.
"In the games I've watched, both domestically and internationally, I've not seen anything in Costa that makes me think, 'you are unbelievable'," he told talkSPORT.
"I know he's been injured, and has probably been rushed back, but I haven't seen a spark, that sharpness or any sort of finishing quality. Even if you're injured you hit the target."
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