I'll be honest, one person certainly lowers a team's performance, but one person can't change the outcome.
Football is a team game, and placing all your trust in one player is like shifting responsibility to just one person. I don't agree with this vision of the game.
Stars make the difference, but let's remember that we're all important; no one is indispensable. Atalanta just needs to find a good goalscorer, perhaps one better than Retegui. There are many young talents yet to be discovered, and there always will be, we just need to give them opportunities.
This is why most teams struggles a lot anytime that their game changer is not playing well or got injured. Am equally not a big fan of depending on a single player because once they have been held down, the team will be unable to perform as they should have done.
And for Atalanta to get back to the strong team that they are known to be, they have to add more firepower to this team and play collectively, than depending on a single player to do the magic, with that they might even perform better than what they did last season. As for the case of ademola lookman, I don't really know how his relations with the team is, if he will be reintegrated into the team or not, since his plan on moving away didn't materialize, but since the transfer market is closed, everyone needs to keep their grievance aside and work for the betterment of the team, he has to be brought back into the team because they needs his creativity.