D'Arcy partners Eoin O'Malley at centre, as Fergus McFadden switches to the left wing in a back-three which again includes Shane Horgan and Isa Nacewa.
Mike Ross starts in a pack containing a mix of youth and experience. Devin Toner, the province's latest Ireland international, starts in the second row alongside skipper Leo Cullen, with Rhys Ruddock forming an exciting back row combination with Dominic Ryan and Sean O'Brien.
Leinster can call upon experience from the replacements bench with the likes of Shaun Berne, Nathan Hines and Shane Jennings all included.
Previewing Friday's game, Ruddock said: "The two challenges facing us this week were the short five-day turnaround and obviously the poor weather which has struck the province.
"Because of the snow, training on the field has been limited, but because of the close proximity of matches we mightn't have done an awful lot of work on our feet anyway, so we just got on with our preparations.
"It meant (that) there was one less training day, but we still had our day off after the Ospreys game and we approached our analysis and preparation as we would any other game, so it's not an excuse.
"December's a big month for the province and we're all excited by the challenges ahead. We're finishing off this part of the league season before two big European games, but we're fully focused on this game. Nothing else matters.
"You want to be involved in all of the games and put yourself in the picture (for selection), so Friday night provides us all with another big test and one we're looking forward to."
The 20-year-old flanker added: "Some people might be surprised by the Scarlets' good form this season, but you could see glimpses of the kind of results they could achieve last season and they have carried on from there.
"David Lyons is a big player for them and someone we'll have to target because he can dominate play if he's given time and space.
"Regan King carries a strong attacking threat and they try to play a fast, offload game which can hurt you. Their game-breakers give them good 'go forward' ball, so they'll be difficult opponents on home soil."
LEINSTER (v Scarlets): Isa Nacewa; Shane Horgan, Eoin O'Malley, Gordon D'Arcy, Fergus McFadden; Jonathan Sexton, Eoin Reddan; Heinke van der Merwe, Richardt Strauss, Mike Ross, Leo Cullen (capt), Devin Toner, Rhys Ruddock, Dominic Ryan, Sean O'Brien.
Replacements: Tom Sexton, Jack McGrath, Clint Newland, Nathan Hines, Shane Jennings, Paul O'Donohoe, Shaun Berne, David Kearney.