Pienaar is close to returning to fitness and could feature in Friday's RaboDirect PRO12 clash in Glasgow - but Marshall remains confident he can do enough to prove he is worthy of a place in Declan Kidney's squad.
"Certainly Ruan is a great player, the most high-profile player we've ever had at Ulster," said Marshall.
"There were times last year when he got us out of jail on a few occasions. But I know I need to take every opportunity, because I want to see myself playing for Ulster and I have ambitions beyond that as well.
"I want to play for Ireland and that should be every player's goal. If you look at the scrum-half position in Ireland now compared to three or four years ago, there's a lot more competition.
"I need to be selfish because I want to wear that green jersey as well, but I need to make sure I'm putting in performances for Ulster first and foremost."
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