Robbie Henshaw is a late withdrawal having failed to fully recover from a minor hip flexor injury picked up in last week's Ireland game.
Having played in every game this season, last week's try scorer Jack Carty has been handed the reins at outhalf.
Along with Ah You coming in, there are three more changes to the forwards pack. Hooker Tom McCartney, who earned his first Connacht cap from the bench last week, has been named for his first start in the absence of the injured Jason Harris-Wright.
The Galway-born duo of Andrew Browne and Eoin McKeon has also been promoted from the bench for tomorrow night's clash at the Sportsground.
Commenting on the team selection and the upcoming game Head Coach Pat Lam said
"We were all very disappointed for Jason Harris-Wright this week but we're thankful to have the experience of Tom McCartney to step in for his first start in a slightly changed pack.
"Scarlets are a well-coached side with huge squad depth and it's been over three years since we beat them. Their win last week over an in-form Glasgow Warriors highlights the big challenge we face tomorrow night. But with both teams vying to climb the table, it is set up for a great evening at a lively Sportsground."
Connacht team to face Scarlets
15. Mils Muliaina
14. Danie Poolman
13. Bundee Aki
12. Dave McSharry
11. Matt Healy
10. Jack Carty
9. Kieran Marmion
1. Denis Buckley
2. Tom McCartney
3. Rodney Ah You
4. Quinn Roux
5. Andrew Browne
6. John Muldoon (captain)
7. Eoin McKeon
8. George Naoupu
Replacements:
16. Dave Heffernan
17. Ronan Loughney
18. Finlay Bealham
19. Mick Kearney
20. Willie Faloon
21. John Cooney
22. Craig Ronaldson
23. Darragh Leader
Unavailable/Injured: Jason Harris-Wright, Sean Henry, Nathan White, JP Cooney, Aly Muldowney, Jake Heenan, Robbie Henshaw, Tiernan O'Halloran, Niyi Adeolokun.
Connacht v Scarlets
at the Sportsground, 7.30pm.
Live on SKY SPORTS and TG4
Referee: Leighton Hodges (WRU,43rd competition game)
Assistant Referees: Nigel Correll, Brian MacNeice (both IRFU)
Citing Commissioner: Eugene Ryan (IRFU)
TMO: Seamus Flannery (IRFU)
Tickets are selling fast for Connacht v Scarlets but there are some remaining for the Main, Enclosed and new Seated Stand as well as behind the goals. Starting at just €20 for adults, tickets are available from tickets.ie, Centra & SuperValu stores province-wide or from the Connacht Rugby office at the Sportsground.
Guinness PRO12
Suite 208, Alexandra House,
The Sweepstakes
Ballsbridge, Dublin 4, Ireland