The £85million summer signing from Benfica has scored nine goals in 20 appearances in all competitions, but has been criticised by pundits and fans for missing lots of chances.
According to the Uruguayan Football Federation, O Neill, who had 19 caps for the national side, was admitted to hospital on Saturday in a coma after suffering a hemorrhage.
Fernando Santos side finished top of their group on Friday, but were beaten by South Korea, with Ronaldo at fault for the first goal that his team conceded in their 2-1 loss.
Uruguay managed to beat Ghana in their final group game on Friday, but a late winner from South Korea against Portugal in the other match in Group H meant that Alonso s side exited the tournament.