Replacing him might not be that big of an issue. For a start, he has only been here one season and the team will no longer have to cater to him as it currently does. This gives ETH a degree of flexibility.
We certainly need a new attacker who will hopefully benefit the team more than Ronaldo did. Someone who will press, defend from the front and play for the team and not just himself. Ronaldo scored 18 league goals last season, almost half of which came in just three games, his contribution was over rated in my view. I'm sure we can manage to sign someone who can score 15 league goals and has higher work rate.
Carragher was right when he said last year than Ronaldo would have a great season but United will be poor.