But 25-year-old Marshall has been in fine form since filling the boots that Isaac Boss vacated in the summer.
Marshall has been one of the stand-out performers in Ulster's 100 per-cent winning start to the season and was named man-of-the-match in their 29-21 victory over Edinburgh at the weekend.
And the Ireland A international has vowed he will not simply roll over and let the 43-times-capped Springbok walk into the side.
"Ruan Pienaar is a world-class player and there's no hiding from that fact," Marshall told the Belfast Telegraph.
"But the key thing for me during the week was to focus on my job against Edinburgh.
"What happens next week or the week after that is out of my control."
Marshall revealed Ulster backs coach Neil Doak took him aside before kick-off to give him some quiet words of encouragement.
"Neil said to me at the team run just to keep my head down and concentrate on the game in hand so that's what I tried to do," added Marshall.
"At the end of the day, Ruan is coming into the squad and he's going to play some rugby for Ulster at some stage.
"I've just got to keep my head down and do my best any time I get a chance."
Although Pienaar has stated his desire to play scrum-half this season, the South African's versatility may work in Marshall's favour.
Pienaar has featured in every position bar centre for his country and that varied skill set could yet see him feature in the same team as Marshall at some point this term.
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