Real Madrid won't be left behind for sure. There's no competitor to Real Madrid in La Liga even without Mbappe. There are some injury concerns that are affecting the team but they are not permanent and Real madrid would soon rise to the top spot.
Mbappe is indeed in pressure to make him stand out as a promising player. La Liga is a big platform to show your skills and many people believe Mbappe is the new Ronaldo or Messi as both of them have left the European Leagues but Mbappe is not being able to rise to the fame or Earling Halland.
Champions League is another tough platform where Real Madrid would have some competition and its where Real madrid and Mbappe has to prove themselves.
Well I did check the players currently injured in Real Madrid and let's say maybe injury could be the cause,I can count upto 4 key players from different positions currently on injury but at the same time their main rival (Barcelona) has been facing alot of injuries too from last season though it affected them alot last season...
About Mbappe, I agree more that he's indeed pressured, I don't know what everyone is expecting from him 10 goals in a game?? though he needs to work on himself but the pressure of miracle goals is too much on him.