It was the same with United during that albeit brief period when van der Sar was injured in 2009/10. Ben Foster was and is capable of world class performances, but he's also extremely prone to very costly errors completely out of nowhere, and when your defenders are shitting themselves about every attack, it has a knock on effect.
All goalkeepers are prone to some mistakes, because all players in any position are. The difference with goalkeepers is that their mistakes more often than not, lead to the team conceding.
Alisson has made a couple of errors this season, but fortunately for you, they've not mattered. It'll be interesting to see what happens if he does end up making a costly error, but until that day comes, you've nothing to worry about really.
You've benefited similarly from van Dijk's presence in the defence, because it means your young defenders are no longer looking at Lovren for guidance and leadership in the back-line. It's that sort of presence that United need at the moment, because none of Bailly, Lindelof or Shaw are ready to lead a defence, and with the best will in the world, none of Jones, Smalling, Rojo, Valencia or Young have the ability or command you want from your defensive leader.
It's mad how we've still not replaced Vidic, Ferdinand and Evra in that regard.