Like so many players, he has not been given a run in the team, even when he played well in the first game. It can't be a coincidence that we keep signing players that have contributed to their team on a world level, yet "fail" here after not being given a chance to play a run of games in the premier league. It is impossible to adjust if you don't play - even the "manager" commented on players signed from South American countries that they take six months to adjust - it is even harder if they don't play, if Bruce started backing all his signings and at least played them for a good run, they may fair better. For example, due to injuries Sess has had a good run, if he had only had odd games over a season we may not have seen the potential he has displayed.
For the record, I think Riveros has shown plenty of promise on limited appearences and he deserves a run in the team next season...but if he isn't going to give him a good run, he may as well get rid of him.