The injury crisis that has thinned the Irish ranks has seen the uncapped Ulster duo thrust into a pivotal match on which the team's flagging title aspirations hang. And while the 21-year-olds' potential is matched by their inexperience, Heaslip is backing them to make a swift adjustment to Test rugby.
"When they played against Fiji in November they showed what they can do and how professional they are," the Lions number eight said. "I'll give them advice about nerves, but now the best advice is just to go out and play your game. That's why they were picked. There are 14 other guys on the pitch who are ready to die for you so go out and play."
He went on: "At international level, the space is that little bit less than Heineken Cup. The margins get smaller.
"But I've 100% faith in Luke and Paddy when it comes to dealing with that kind of pressure.
"They've been picked because they're ready and good enough. They must back themselves and if they see a chance, take it."
Ireland's chances of winning the Six Nations were damaged by their 12-6 defeat to England and they must prevail in Edinburgh if they are to remain in the title hunt.
A loss would result in another failed championship and weaken coach Declan Kidney's chances of extending his contract with the Irish Rugby Football Union until the 2015 World Cup.
The selection of the untested Jackson and Marshall - potentially two stars of Irish rugby - at fly-half and inside centre has heightened the significance of the occasion.
"Murrayfield is a great stadium. In a weird way I have mixed memories about it," Heaslip said. "We won the Heineken Cup there with Leinster and two Ireland internationals, but also lost one international before the 2007 World Cup. Edinburgh's a great place to be for a Six Nations weekend. There's a great buzz."
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