While some Los Blancos fans may have been disappointed, Morientes - formerly of Madrid and Monaco - believes that the Liga outfit did not need Mbappe after a successful season where they won the league and the Champions League.
"Given the fact that Real had an exceptional last season, there was no need to buy too many players," he said at an event in Abu Dhabi.
"Usually, when you are in need of bringing a new player, you [buy] at the end of a bad season. In my view, Real have done the right thing by not opting for a major change.
"Mbappe is a fantastic player and he has an opportunity to take his game to another level. Only then will he get a chance to play in a team like Real Madrid.
6 - Kylian Mbappé has scored in six of his last seven games in the Champions League. Astounding. pic.twitter.com/1CRO0Ksm59— OptaJean (@OptaJean) September 12, 2017
"To play for Madrid, you really have to be at the top of the game and should be one of the best around."
Morientes feels that PSG will be in the running to take Madrid's Champions League crown, but adds that they have some way to go before they are better than Zinedine Zidane's side.
"PSG are clearly one of the favourites to win the Champions League," he added.
"But, to reach the level where Real Madrid are, you not only need good players but also need to make sure they play as a team. We have to wait and see if they can do that."