I think the issue being that Sane is now out for the best part of 2 months and we don't want our season de-railing or being forced into a tactical switch. I like Bernardo, but he's not a natural wide player - probably more of a long term replacement for Spanish Dave, but will do a job out wide. Mahrez will hug the wing and play that position more effectively imo.
Also with Jesus and Sane both out at present, it makes sense to have back up in that position, otherwise we're threadbare if we get another long term injury to one of our attacking players.
Think Mahrez would be a good move. Proven at premier league level and technically a fantastic player. Think anything in excess of £60mil would be too much mind you, but can't help the current market.