Ball injured a foot during the latter stages of the Six Nations campaign, but has recovered for the crucial derby clash that will decide which Welsh side competes in the top tier of European rugby next season.
The second row is one of four personnel changes, all of them up front, from the side that took the field in the Scarlets’ last outing against the Dragons on Judgement Day.
Injuries to Rob Evans and Wyn Jones sees Phil Price start at loose-head, although Jones does feature on the bench.
On the other side of the scrum, Samson Lee is ruled out because of concussion so Werner Kruger steps in at tight-head prop.
Ball partners Steve Cummins in the second row, while in the back row Blade Thomson makes his first start since November and wears the No. 8 jersey. Josh Macleod switches to blindside with Will Boyde starting at openside.
Wales internationals Aaron Shingler and James Davies have failed to recover from injury, but hooker Ryan Elias returns from a shoulder issue to take his place on the bench.
Scarlets head coach Wayne Pivac said: “We have a proud history in the top echelon of European rugby and clearly that is where the club would like to be again.
“We have prepared well and we have all talked about the importance of the game.
“Ordinarily, the Ospreys is one of the big fixtures of the season so to have this added game, there is a lot of motivation for both sides.”
SCARLETS: Leigh Halfpenny; Johnny McNicholl, Jonathan Davies, Hadleigh Parkes, Ioan Nicholas; Dan Jones, Gareth Davies; Phil Price, Ken Owens (capt), Werner Kruger, Jake Ball, Steve Cummins, Josh Macleod, Will Boyde, Blade Thomson.
Replacements: Ryan Elias, Wyn Jones, Simon Gardiner, David Bulbring, Uzair Cassiem, Kieran Hardy, Rhys Patchell, Paul Asquith.
Unavailable because of injury: Rob Evans (shoulder), Kieron Fonotia (knee), Aaron Shingler (knee), James Davies (hamstring), Dylan Evans (hamstring), Rhys Fawcett (shoulder), Samson Lee (concussion), Tom Phillips (shoulder), Jonathan Evans (ankle), Lewis Rawlins (shoulder), Dafydd Hughes (shoulder), Angus O’Brien (knee), Corey Baldwin (ankle).