Henry sprained his ankle in the RaboDirect PRO12 game against Leinster last Friday, but will be given until late in the week to prove his fitness.
Marshall, who injured his elbow against Connacht the previous week, will also be reassessed throughout the week before a decision is made.
The Ulster management team will have a near full strength squad to choose from as Pedrie Wannenburg (groin) and Paddy Wallace (illness) are likely to be available for selection despite also being forced off during the Leinster game.
Edinburgh's form in Europe, in which they have beaten the likes of Racing Metro 92 and Toulouse, is in stark contrast to their league form, where they have picked up just five wins in 21 outings.
Ulster have already met Edinburgh in the RaboDirect PRO12 twice this season, with McLaughlin's charges winning 42-20 away and 38-16 at home.
Edinburgh are appearing at the semi-final stage for the first time in the club's history and they are indeed the first Scottish club to qualify for the last-four.
Ulster's last appearance at this stage of the competition was on route to winning the trophy in 1999, when they beat Stade Francais 33-27 at Ravenhill.
The sides will be competing for a place in the final at Twickenham on May 19, where the winners will face either reigning champions Leinster or French giants Clermont Auvergne.
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ULSTER Squad (v Edinburgh) - Backs (16): Blane McIlroy, Ruan Pienaar, Ian Porter, Paul Marshall, Ian Humphreys, Paddy Jackson, Paddy Wallace, Ian Whitten, Darren Cave, Luke Marshall, Nevin Spence, Stefan Terblanche, Craig Gilroy, Conor Gaston, Andrew Trimble, Adam D'Arcy.
Forwards (19): Callum Black, Tom Court, Declan Fitzpatrick, Adam Macklin, Paddy McAllister, Andi Kyriacou, Nigel Brady, Rory Best, Lewis Stevenson, Johann Muller (capt), Dan Tuohy, Iain Henderson, Neil McComb, Stephen Ferris, Willie Faloon, Pedrie Wannenburg, Mike McComish, Chris Henry, Robbie Diack.