Scarlets remain in the hunt for the URC play-offs, coming into the match in ninth place, two points off the top eight qualifying spots.
Runaway league leaders Leinster have lost just once this season – by a single point at Bulls in March – and have not been defeated by a Welsh region since a visit to Cardiff in January 2022.
Scarlets have not beaten an Irish opponent since a 40-36 win at home to Connacht in March 2021.
Head coach Dwayne Peel has named Costelow in a side which shows four changes from the 31-23 comeback victory over the Dragons last weekend.
Costelow has recovered from the shoulder injury he picked up against Gloucester in January, while centre Johnny Williams returns after overcoming a calf issue that has sidelined him for the last three matches.
Hooker Ryan Elias is the third change, switching with Marnus van der Merwe and packs down in between props Alec Hepburn and Henry Thomas.
Departing Scottish lock Alex Craig continues with last weekends man-of-the-match Sam Lousi in the second row, whilst Vaea Fifita, Taine Plumtree and skipper Josh Macleod form an unchanged back-row.
An all-new midfield sees Williams partnered by Joe Roberts, with Mags Page dropping to the bench and Eddie James missing out completely.
The all-international back three of Blair Murray, Tom Rogers, and Ellis Mee remains the same, with Costelow replacing Ioan Lloyd to link up with Gareth Davies at half-back.
“It’s going to be a big occasion against the team in the best form in world club rugby,” said Peel in his pre-match press conference.
“They are a fantastic side. They have won 20 out of 21 this season and you’ve seen how good they’ve been over the last few weeks.
“But I don’t see it as a daunting thing. It’s obviously going to be a massive challenge, but it’s also an opportunity for us to have a go at them and try and make them uneasy.
“You’ve got to try and upset the rhythm of their game and get in amongst them. You’ve got to be brave and you’ve got to be bold. You have got to be smart as well.
“Their defensive set-up is very aggressive. They come off the line and they want to hit you behind the gain-line.
“They are very intense defensively. You look at their last two European games against Harlequins and Glasgow – two pretty efficient attacking sides – and they nilled them both.
“So you’ve got to play in the right areas, you’ve got to be accurate and you’ve got to be as physical as you can be.
“I want to see us show a bit of character and try and upset them the best we can. You want to test yourselves against the best and we are excited about it.”
“It’s so congested between fifth and 12th. There’s still a lot of rugby to be played, with sides playing each other, and I wouldn’t be surprised if it goes down to the last week.”
Scarlets XV to face Leinster:
Blair Murray, Tom Rogers, Joe Roberts, Johnny Williams, Ellis Mee, Sam Costelow, Gareth Davies
Alec Hepburn, Ryan Elias, Henry Thomas, Alex Craig, Sam Lousi, Vaea Fifita, Josh Macleod (captain), Taine Plumtree.
Replacements:
Marnus van der Merwe, Louie Trevett, Sam Wainwright, Jac Price, Jarrod Taylor, Archie Hughes, Ioan Lloyd, Macs Page.
[Lead image: Scarlets Rugby]
