McCusker starts after a disciplinary hearing this week upheld the red card issued to him in the Boxing Day defeat to the Ospreys.
He starts alongside Josh Turnbull and Johnathan Edwards in the back row for the mouth-watering PRO12 game at Ravenhill.
The Scarlets are nine points behind leaders Ulster who boasted a perfect record so far this season before their defeat to Munster on Saturday.
With just three points separating the Scarlets from Leinster in sixth place, a win would see them keep up the pressure at the top of the table. Head coach Simon Easterby though, is focusing solely on tomorrow’s game.
“We know it’s going to be difficult to stay in the top four but we need to get things right in our game this weekend to start with,” he said.
“There’s a strong team ethic in our squad and with the quality and experience we have in our ranks, it’s a huge opportunity this weekend.
“Ulster have an impressive record at home and they are bringing back their top players in front of a home crowd after resting a big group last weekend.
“They’re in a really good place and are a difficult team to play against. We showed in early December that we can create decent opportunities against them, this time we need to take advantage of that.”
Ulster triumphed 19-12 at Parc y Scarlets in December and the Irish side haven’t been beaten in a PRO12 game at Ravenhill since 2011.
After demanding games with the Blues and the Ospreys over Christmas, Easterby is without a number of key men. Welsh internationals Matthew Rees, Ken Owens and Aaron Shingle are all out while full back Liam Williams has failed to recover from concussion suffered at the Liberty Stadium.
Dan Newton starts in place of Williams with Emyr Phillips at hooker.
Wales centre Jon Davies captains the side for a fifth consecutive match and knows the Scarlets need to hit the ground running.
“We’ve given teams easy points early on in games against opposition that can close games out,” he said.
“Ulster have a high intensity game plan, they will come at us with a solid game plan and we’ll need to start better and play in the right areas.
“We are playing well in patches but we have to find that for the full 80 minutes.”
