Rafa Benitez's team have won only once in the last nine games, and are now closer in terms of points to the relegation zone than the top of the table.
Riera said: "In games like the one against Birmingham you could see we were trying to do everything," said the player who suffered a recurrence of his hamstring injury which is likely to rule him out for more than a month.
"We played wide, between the lines, always going forward and only defending with two centre-backs. We were really unlucky, because they had one free-kick and one shot from a long way out.
"We are trying to play like we have always done but you can see it is not working. We have been really unlucky this year, because of all the injuries and because we have conceded a lot of goals from set-pieces. You did not see that last year and we are the same players, working in the same way.
"Now our problem could be mental, because we are being untidy, as much as I do not like to say that. We have plenty of possession, plenty of opportunities, and while it is clear we have to manage conceding goals from free-kicks, we should be winning games like the Birmingham match."