Ashton, who started Saints' Pool One victory over the Scarlets last weekend, does not even make the replacements' bench.
There have been reports of a fall-out between Saints rugby director Jim Mallinder and Ashton, whose move to Northampton's Aviva Premiership rivals Saracens next season was confirmed by the Hertfordshire-based club on Thursday.
Saracens said Ashton, 24, had agreed a "long-term contract," which ended speculation about his future next term.
In announcing their team for Saturday's game, Saints gave no explanation for Ashton's absence, and there was no mention of him in their official club press release.
Jamie Elliott replaces him on the right wing, with three other changes from the Scarlets fixture seeing starts for centre James Downey, fly-half Ryan Lamb and scrum-half Lee Dickson.
It now remains to be seen how much more game time Ashton will receive with Saints this term.
He is expected to start England's RBS 6 Nations opener against Scotland at Murrayfield a fortnight on Saturday, but he could now find his opportunities with Northampton severely restricted.
"We had a good result at the Scarlets last week, and if we attack with the same intensity and defend with the same passion and determination then we will have a good chance against a tough Munster outfit," Mallinder said.
"We are definitely going into tomorrow's match with our minds focused very much on getting the win.
"We have had a good run recently and have some big matches coming up. We want to keep our momentum going."
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