Cusack is included for the first time since featuring in the defeat to the Dragons at Rodney Parade in February 2014. It will be his first appearance at Scotstoun since November 2012.
The Scottish qualified prop has battled injury and illness over the past two seasons, but his return is a big boost for the Warriors.
Following the defeat to Edinburgh Rugby at BT Murrayfield last week, Peter Murchie keeps his place at full-back and Tommy Seymour is retained on the left wing, while Sean Lamont comes in for Sean Maitland on the opposite side.
The centre partnership remains the same, with Peter Horne at inside centre and Alex Dunbar outside him.
It's also the same half-back pairing from last week, with Finn Russell at stand-off and Niko Matawalu at scrum-half.
In the pack there is a new front-row, with Jerry Yanuyanutawa, Pat MacArthur and Cusack, coming in for Gordon Reid, Dougie Hall and Jon Welsh.
Tim Swinson replaces Al Kellock in the second-row and he'll partner Jonny Gray.
Ryan Wilson moves across the backrow to play at openside flanker, replacing the injured Tyrone Holmes. Leone Nakarawa starts at blindside flanker, with Josh Strauss captaining the team from Number 8.
The Warriors have won their last 14 GUINNESS PRO12 fixtures at Scotstoun, since Newport Gwent Dragons won there in November 2013. Glasgow's solo defeat to a Welsh opponent in their last six such encounters came on a trip to the Scarlets in this return fixture in November.
Tickets are still available for the GUINNESS PRO12 clash from www.glasgowwarriors.org and from the ticket office until 5pm today on 0844 800 3490. Adult and concession tickets will increase by £5 on Friday and tickets will be available at the stadium from 12pm on matchday (subject to availability).
Glasgow Warriors head coach Gregor Townsend said: "We were disappointed with the performance and result last weekend and the players have been hurting since the game at BT Murrayfield.
"Tomorrow is a great opportunity for them to deliver a much improved performance and show our loyal supporters how hard they are willing to work to bounce back with a win.
"We face a tough challenge against the Scarlets, who are very well-coached and have a number of quality players in their side.
"It's great to be able to welcome back Mike Cusack for tomorrow's match. He's shown a lot of courage over the last couple of seasons and his hard work has been rewarded with a starting place in the team."
Glasgow Warriors team to play the Scarlets, GUINNESS PRO12, Scotstoun Stadium, Friday 9 January (kick-off 7.35pm) - live on BBC ALBA
15. Peter Murchie
14. Tommy Seymour
13. Alex Dunbar
12. Peter Horne
11. Sean Lamont
10. Finn Russell
9. Niko Matawalu
1. Jerry Yanuyanutawa
2. Pat MacArthur
3. Mike Cusack
4. Tim Swinson
5. Jonny Gray
6. Leone Nakarawa
7. Ryan Wilson
8. Josh Strauss (Captain)
Replacements
16. Fraser Brown
17. Ryan Grant
18. Euan Murray
19. James Eddie
20. Will Bordill
21. Ali Price
22. James Downey
23. DTH van der Merwe
Not available due to injury/illness: Adam Ashe (neck), Mark Bennett (leg), Kevin Bryce (shin), Rossouw de Klerk (foot), Chris Fusaro (ankle), Rob Harley (leg), Stuart Hogg (leg), Tyrone Holmes (eye/face), Lee Jones (ankle), Murray McConnell (leg), Henry Pyrgos (knee), Tom Ryder (neck), Richie Vernon (illness) and Duncan Weir (arm).
Guinness PRO12
Suite 208, Alexandra House,
The Sweepstakes
Ballsbridge, Dublin 4, Ireland