The Brazilian played every minute of the Reds' Community Shield loss against Manchester City on Saturday, as we drew 1-1 with Pep Guardiola's team but were eliminated on penalties.
Casemiro was impressive in midfield throughout, with Erik being asked about the United no.18 in his post-match press conference.
"He has a pre-season so he is very fit in this moment," said the boss.
"I think, as a team, we work good together and I think that makes it for Case very easy. Then he can bring his skills in and then you can see his importance for the game and for our team.