Rees has pinpointed that defeat as one of the main disappointments in the Scarlets' season which showed so much early promise.
The Welsh region still have a chance of reaching the Magners League play-offs, five points adrift lying sixth in the league table.
And Rees has called on his side to move a step closer to the play-offs starting with a win over the Ulstermen.
"We put ourselves in a great position in beginning of season," said Rees.
"The frustrating thing is that Ulster game and the late kick from Pienaar from the halfway line that took that result away from us.
"Also the draw with Leinster, if we had won those two we would be second or third now. So we owe Ulster one and it's great we have got that opportunity.
"It's a tough ask to go to Ravenhill and get a result. But we'll have our best team out and Ulster are looking forward to a big European quarter-final, it is a big challenge but it's in our hands.
"We have four games remaining and they're all must win. Munster are out of the Heineken Cup and will be looking to finish top of the Magners with a home draw.
"It's going to be a bit of a dogfight between the other teams. But if we can start off well against Ulster with a victory it's in our hands."
Head coach Nigel Davies has also pinpointed the threat of the Springbok halfback but insists there is a lot more his side have to worry about.
"He's had a massive influence on their results this season and is a key player for them," said Davies .
"At big moments he's a danger but Ulster are about more than that.
"We failed miserably at the lineout against them in the last came and they will come at us in that area again. The kicking game will be important and the contact area is always crucial against Irish sides.
"They have the knack of staying in games, they do that particularly well and when that happens they have the likes of Pienaar to take the opportunities."
Rees returns to captain the side but Davies is without teen sensation George North, who recently signed a new three year deal with the region, and Scotland centre Sean Lamont. David Lyons has recovered from injury but is restricted to a place among the replacements.
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