Riera has had a good first season with us IMO. He was thrown straight into the squad after only joining days earlier, so from that first point of view, he has done well.
You also have to look at the amount of games he played. The previous season at Espanyol he played a total of 44 games, for just his club and 1 international, alot of those games were in the UEFA cup when they got to the final. La Liga is a much slower pace as we all know, but I'm pretty sure he was knackered after that season.
Now look at his first season with us, he played 40 games, but they included big european games and fast paced, tough games in the prem. He also broke into the national squad again and had to play in world cup qualifying games, including travelling to Turkey for one.
Now I know it sounds like a poor arguement, but the guy was thrown into the team without anytime to settle in, it was a new league that's alot more physical and faster than the one he was used to, he also had to go away on international duty, time which he probably used to use to relax and recharge his batteries.
I'm sure after his experiences this year, he will come back better and stronger next season, knowing exactly what he has to do to play at the top.
Besides, I love him, so leave him alone!!!