A club resting the success of the club on the shoulder of a particular player is bound to fail if the player decides the leave the club for the other. Mane's departure from Liverpool is a prime illustration of this.
So many other clubs have such mistakes in the past, depending on a player for winning. it took them time before they could rebuild and bounce back to their normal state of winning.
We observed how Liverpool experienced that. They refused to let Mane's leaving stop them from at least making it to the Europa League.