Atletico will defend a first-leg 1-0 lead at Liverpool for their Champions League round 16 tie.
Riera said: "I know what Anfield is like, it will be very hard.
"The 90 minutes, on that narrow pitch, where the fans crowd in, and with how Liverpool are right now…
"Although they are not at their best, it is very difficult to beat them. In the first leg, Atletico managed to score and defended well. But it will be very difficult [for them at Anfield]."