Edinburgh were defending their line, as they were for much of the game, when Rizzo decided to do some over-enthusiastic clearing out on home scrum-half Gareth Davies.
What turned it from a probable penalty to the sending off was that he piled into the head of Davies. He may not have intended it maliciously but in this age of sensitivity over the head region it was always going to be a red. It was not the sort of incident that looked any better in the slow motion replays on offer to Irish referee John Lacey.
Edinburgh had battled, with captain Magnus Bradbury leading from the front, but had been outclassed even before they found themselves a man down.
If last week was old Edinburgh, slipping to a sloppy defeat after raising hopes and expectations by winning their first two games, this week was about new coach Richard Cockerill stamping his mark and showing there would be new Edinburgh under him.
Travelling to face the champions was certainly a tough assignment, but he wanted a reaction and sent that message clearly through the selection with seven changes from the side that lost at home to Treviso. And one player who was clearly particularly keen to impress was left wing Jason Harries, from Scarlets country in Carmarthen and having arrived in Scotland via Wales Sevens and London Scottish.
The Scarlets were attack minded and eventually it paid off. They worked some space down the right hand side, but it seemed the Edinburgh defence had coped once again – even when they conceded a penalty through full-back Blair Kinghorn trying to win the ball in the ruck.
However, Scarlets scrum-half Gareth Davies took the quick tap penalty and almost made it to the line, more quick ruck ball allowing outside half Rhys Patchell spot a gap to go over for the try, converted by Leigh Halfpenny.
The Scarlets kept coming, Patchell mixing in the occasional kick to gain territory and maintain the pressure. Another series of rucks won in the Edinburgh 22 stretched the defence enough for Davies to score himself, racing through from ten yards out.
Edinburgh needed a chance to maintain some pressure of their own and they did earn a penalty in front of the posts 35 yards out which stand-off Duncan Weir was able to put over to get his side on the scoreboard.
Better was to come just before half-time and Harries would earn some reward for his work. Edinburgh took a leaf out of the Scarlets attacking book and worked it wide to Stuart McInally and Hamish Watson.
They made ground and when the ball went back inside Harries, inset, came in on a good angle to burst to the corner. Weir missed the touchline conversion, but still Edinburgh were given renewed hope for the half-time interval.
It did not take long for that hope to be snuffed out. The Scarlets were straight back on the attack near the Edinburgh line when Rizzo launched his attack on Gareth Davies, the Scarlets scrum-half leaving the field as a result.
Edinburgh were not only down to 14, but from that penalty they ended up conceding the third try as replacement prop Werner Kruger burst through the re-organising forwards for the score. Halfpenny converted.
The Scarlets had their tails well and truly up and though Edinburgh did launch a period of pressure of their own, they were soon back on the defensive.
Superb Scarlets handling worked the ball out to the left where flanker Aaron Shingler finished off the move with the try and the bonus point, again Halfpenny converted.