They face Munster at Thomond Park in a battle to be crowned Magners League champions but will be without influential Ireland centre D'Arcy.
The Lions centre picked up a ankle injury as his side stormed back against the Saints to secure European glory and has been ruled out of this weekend's clash.
"We have a couple of injuries and Gordon won't be available for selection," said Leinster head coach Joe Schmidt.
"He hurt his ankle in the Heineken Cup and has been ruled out.
"There are a couple of others with niggles but hopefully they will come through. Isaac Boss isn't looking in great shape and is also a doubt."
But there are no concerns over D'Arcy's midfield partner Brian O'Driscoll. The Ireland captain and three-tour Lion was a doubt ahead of the Heineken Cup showdown with a knee injury of his own but he came through the game unscathed and back firing on all cylinders.
The fitness of O'Driscoll is a major boost for Leinster with Schmidt adamant Munster will provide a tougher test than Jim Mallinder's men.
Schmidt added: "There's an air of expectation and hopefully we can replicate our second half performance against the Saints.
"But no disrespect to Northampton, I cannot see Munster giving us as many opportunities. Especially at Thomond Park Munster will have a lot more resolve and will be difficult to break down.
"Northampton were tired, maybe it was the cumulative impact of a long season, but Munster won't offer us those opportunities in attack."