I don't think it's as much that he'd struggle without Casemiro as that whoever played instead of Casemiro would. Partly because Casemiro is one of those rare players you just can't replace, and partly because our other midfielders just aren't that good...or have just arrived on loan from barely playing in a completely different league.
I don't think that means Eriksen is "too lightweight" to play in midfield as this thread literally suggests. I mean that just quite clearly isn't true as we've had a very good midfield all season and win nearly all our games, and apart from against City and Arsenal, look like the better team in the ones we don't. What people don't seem to notice with Eriksen is he does the same job Fred/Mctominay do, he just also knows how to play a forward pass and as a result not be caught ahead of the play every time we lose the ball.
The problem is more, as we're now finding out, as soon as Eriksen needs a rest or is taken off, we are back to having half a midfield and ending up with the opposition dictating the game, because what Fred/Mctominay tend to do is come forwards with the ball and then go sideways, and then end up behind the play as soon as we lose it. Either that or they'll just give it straight back to the opposition.
As good as Casemiro is, Eriksen is much better at finding space, playing on the turn and getting the team on the front foot by bringing our forwards into the game. Casemiro can do this because he's juat an all round excellent midfielder, but we are going to miss Eriksen a lot more than I think a lot of people realise, which again doesn't really tally up with the idea he can't play in midfield.
I mean I thought our midfield last night was a disaster. No service to the forwards at all, no ability to press or control the opposition because to do that properly you have to be able to receive the ball and pass it ahead of yourself so you are actually in a position to press. No anything really other than a lot of frantic running around. Back to how it has been for years except now I'm used to us having a pretty good pair of midfielders and, well, actually tactics, I found it quite astounding how much of a mess it was.