Ramires went to ground after a coming-together in the box with West Brom's Steven Reid deep into injury time in the Barclays Premier League game at Stamford Bridge on November 9. Marriner blew for a penalty which Eden Hazard scored to salvage a 2-2 draw for the Londoners.
Marriner, who will take charge of Fulham's home league game against Swansea on Saturday, was heavily criticised for giving a spot-kick when it appeared the Brazilian could easily have stayed on his feet.
Riley said with the benefit of hindsight and numerous replays it was possible to show how decisions such as this one were incorrect, but defended how Marriner reached his decision in the heat of the moment.
"I understand why Andre gave it," Riley added.
"He thinks he sees Ramires getting in front of Reid, catching his back leg and making him off-balance without playing the ball so it's a penalty."
Riley did not judge either way whether Ramires was guilty of diving.
"The truth is only Ramires truly knows," Riley said.
Generally speaking Riley believes players in the English top flight have recognised the moral imperative to clamp down on diving.
Riley cited Premier League statistics to demonstrate that the practice of simulation is in decline, with six incidents identified so far this season compared to 19 by the same stage last term.
"We've seen a drop-off in simulation offences," said Riley, the general manager of Professional Game Match Officials Limited.
"We should be proud it's not an accepted part of our game. Players say it is not acceptable. Supporters said it is not acceptable.
"We mustn't lose the moral argument; it puts the onus on players not to do it."
Yet incidents still occur and officials do not always make the correct decision.
Five key areas must be evaluated in a split-second decision regarding diving - whether or not there is contact; whether any contact is fair or normal; whether the player legitimately avoids contact; who initiated the contact; and, does the attacker exaggerate the effects of a normal challenge.
While Marriner will take charge of a game this weekend, another under-fire official, Robert Madley, misses out.
Madley decided Swansea's Wayne Routledge had handled in the penalty area during stoppage time and Stoke equalised from the spot to force a 3-3 draw a day after the Marriner incident.
Afterwards Swansea manager Michael Laudrup claimed Madley owed him and his players an apology.