It's a pretty standard practice really. Teams get forfeited matches and given a 3 - 0 win to the opponents all the time. This is the garden variety of punishments for fielding an ineligible player. And that's what's on the table.

Now, if things end up being more serious, then there are way more serious consequences.

For example, Chile was banned for two world cups and a player banned for life for El Maracanazo. What a goalkeeper did had extremely harsh consequences for himself and the whole team.