Mils Muliaina has been ruled out for up to a month with a hamstring tear.
Connacht head coach Pat Lam revealed the news as he named his side to face Scarlets in Wales on Sunday (12:45pm).
Eoin McKeon is also expected to be out for four weeks after the back row sustained a fractured sternum.
Kieran Marmion has returned from Ireland camp to take his place at scrum-half for the Guinness PRO12 clash, while Jack Carty starts at out-half.
"We lost Eoin (McKeon) and Mils (Muliaina) to injury which was obviously a big blow especially on top of the loss of another back row in Willie (Faloon)," said Lam.
"This is a huge game for us against a well-coached and quality side in Scarlets. We know they are breathing right behind us and like us, they have top-six aspirations."
Connacht (v Scarlets):
15. Darragh Leader
14. Niyi Adeolokun
13. Danie Poolman
12. Dave McSharry
11. Matt Healy
10. Jack Carty
9. Kieran Marmion
1. Denis Buckley
2. Tom McCartney
3. Nathan White
4. Quinn Roux
5. Aly Muldowney
6. John Muldoon (captain)
7. Jake Heenan
8. George Naoupu
Replacements:
16. Dave Heffernan
17. JP Cooney
18. Rodney Ah You
19. Ultan Dillane
20. Eoghan Masterson
21. Ian Porter
22. Shane O'Leary
23. Tiernan O'Halloran
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