And unsurprisingly, there was little fluency about Liverpool's play early on, with only Riera looking bright and alert.
It got even worse in injury time as Riera's shot struck Babel and looped over Myhill to make it six.
In May, Riera was left out of Spain's Euro 2008 squad despite helping the national side qualify for the tournament.
And Riera drove just wide for 10-men Falkirk in the 70th minute.
Arnau Riera then sent a shot flashing wide following a poorly-cleared corner.
Riera, who spent three years with Barcelona, last played for Sunderland when he was red-carded in the Carling Cup against Bury in August.
Everton are believed to have met Espanyol's asking price for Riera but the player is understood to have favoured a move to Anfield.
And the mission to overturn their first-leg 1-0 deficit was clouded somewhat with Albert Riera's attack on the club's management 24 hours before kick-off.
Arnau Riera came into the side along with the inexperienced Des McCaffrey and Chris Mitchell, who was earning his first start of the season.
Riera's joins Liverpool following the earlier news on transfer deadline day that Steve Finnan was moving in the opposite direction for an undisclosed fee.
Steven Gerrard's cross flew it to make it 4-1 before Ryan Babel tapped in Dirk Kuyt's cross and then unknowingly deflected in Albert Riera's late shot.
But when the goal came, it arrived in emphatic style with the impressive Riera striking a magnificent shot from distance that flew into the top corner.
Ryan Babel equalised with a glancing header from Lucas Leiva's cross and Albert Riera put them ahead with a 25-yard drive that found the top corner.
When the Reds did threaten a goal, Black Cats keeper Fulop kept them at bay as he impressively palmed Albert Riera's deflected shot off Grant Leadbitter wide.
Mr Riera, who is a close ally of Mr Duarte, argues that the Colorados' new leaders have no ties to the past, and are prepared to do the unthinkable.
It seemed to be developing into a familiar story for the Reds but Frenchman Ngog made the vital breakthrough for the home side after Gerrard steered Riera's cross into his path.
The arrival of Ngog in place of Riera gave Liverpool greater presence up front and the Frenchman ought to have opened the scoring when Glen Johnson burst to the byeline and squared from the right.
And Benitez's preparation for the Anfield clash included suspending Albert Riera after the Spaniard, desperate to play in the World Cup, accused his boss of having something "personal" against him and "turning a deaf ear" to players.
Riera came in for the Argentine and the winger had an immediate impact as his low ball in from the left was shifted out of his feet by the quick-thinking Torres before he rolled into the far corner.
Fernando Torres and Glen Johnson both missed Tuesday night's game with groin injuries while winger Albert Riera is also in need of a scan on his hamstring and Martin Kelly's impressive full debut was ruined by an ankle injury.
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