I just haven't seen anything from Díaz to suggest he's anywhere near ready for a regular place in the first team. He hasn't made the step all young forwards need to eventually make, which is learning that dribbling a short distance and then passing is more effective than continuing and then usually losing the ball. He'll get there but I don't think he's there yet. The lad doesn't even have a senior goal for us yet so no, I wouldn't say he's ready to be a viable alternative to one of the best players in the country.
If we can't get Mahrez I'd like to see us go for Zaha.