Now that Benzema is definitely leaving Real Madrid they definitely need to act now to get a proper replacement for him. As you already said, it looks like Mbappe is definitely not available in this transfer period so Real needs to look somewhere else. I am not really convinced, if Sadio Mane would really be a great replacement though.
Real Madrid have doubled their efforts to get a replacement, the start player from Tottenham Hotspur Harry Kane is someone they seriously consider above many. If he leaves Tottenham, the new coach Ange Postecoglou who has just agreed to join them from Celtic on a four year deal will have to make his arrangements for a new striker to replace Harry kane of he leaves the club. He will have his options. Tottenham Hotspur as a team need to improve, some more players may be laid off by the new coach as he will want to bring some new players of his choice to the team.