Marshall and Nevin Spence formed an impressive partnership last season while Cave was out injured and Paddy Wallace was away on international duty, so it should cause minimal disruption to the Ulster midfield.
Ulster are boosted by the inclusion of Ian Humphreys who has been named at out-half after shaking off a shoulder injury picked up in the win over the Cardiff Blues last weekend.
The side is led again by captain Chris Henry at openside flanker, as Brian McLaughlin has stuck with the forwards who dominated up front against Cardiff.
A rib injury has ruled out reserve scrum half Ian Porter, whose position on the bench is taken by 19-year-old Academy player Blane McIlroy.
The only other change to the matchday squad from the Cardiff game is the inclusion of Willie Faloon, who will provide back row cover instead of Mike McComish.
Looking forward to this weekend's showdown in Swansea, McLaughlin said: "The Ospreys have won all their games so far this season too, so it will be another tough game for us.
"If we play as well as we have been I see no reason why we can't get another win and that would put us in a really strong position. We lost by a point to them last season so there's no doubt it will be another big challenge for us."
ULSTER (v Ospreys): Jared Payne; Craig Gilroy, Nevin Spence, Luke Marshall, Ian Whitten; Ian Humphreys, Paul Marshall; Declan Fitzpatrick, Andi Kyriacou, Jerry Cronin, Lewis Stevenson, Dan Tuohy, Pedrie Wannenburg, Chris Henry, Robbie Diack.
Replacements: Nigel Brady, Paddy McAllister, Adam Macklin, Tim Barker, Willie Faloon, Ian Porter, Paddy Jackson, Adam D'Arcy.