Ospreys Director of Rugby Kevin Hopkins said, "When he joined us from Leeds we respected his desire to accept invitations to play at a level as close to international rugby as possible."
The absence of Marshall means that Jason Spice, a starter just twice this season, will wear the number nine jersey this evening.
The Ospreys know they need to shake off the disappointment of their 29-26 slump to Glasgow last Friday. The defeat means they are six points from the top of the table with a game in hand over leaders Leinster.
"We need to quickly move on after last Friday's match", said Ospreys Head Coach Lyn Jones.
"To beat the Dragons we will have to play to our full potential as we did for most of last Friday's match. To get the win will be easier said than done as they are the same as all Magners League teams in that they have a good home record.
"We have never beaten them at Rodney Parade so we will certainly not underestimate the task that awaits us."
Ashley Smith, Lewis Evans, Gareth Chapman, Kieran Crawford (all shoulder), Nic Fitisemanu (hand), Craig Warlow (foot) and Nathan Brew (ankle) are all on the injury list at the Dragons as they prepare for a May 18th Heineken Cup play-off against the third best Italian side.
Prop Rhys Thomas will definitely miss that game and Wales's summer tour, the prop facing three weeks in plaster with a foot injury.
"We hope to put on a real show against the Ospreys, they won't find it easy and don't have a good record at Rodney Parade, but they have to win if they want any chance of the title," said Dragons Coach Paul Turner.
Dragons: Martin Thomas; Gareth Wyatt, Gareth Maule, Phil Dollman, Jimmy Norris; Aled Thomas, Alex Walker; Dai Maddocks, Ben Daly, Jamie Corsi, Andrew Hall, Peter Sidoli, Dan Lydiatt, Joe Bearman, Michael Owen (Capt)
Replacements: Steve Jones, Gethin Robinson/Adam Black, Ian Gough, Jamie Ringer, Paul Emerick, Ceri Sweeney, Gareth Cooper/Gareth Baber
Ospreys: Lee Byrne; Nikki Walker/Jonathan Vaughton, Sonny Parker, Andrew Bishop/James Hook, Shane Williams; James Hook/Shaun Connor, Jason Spice; Duncan Jones/Paul James (Capt), Barry Williams, Adam Jones/Andrew Millward, Brent Cockbain/Mike Powell, Alun Wyn Jones, Andy Lloyd/Lee Beach, Richard Pugh, Jonathan Thomas/Andy Lloyd