Jorgensen warmed the bench last weekend as 20-year-old Chris Hyndman was selected to start. But the centre has been reinstated as first choice for this vital match that will determine which team goes through to the final eight of the competition.
Saints will still be without flankers Andrew Blowers and Budge Pountney, so the pack remains unchanged.
In the backs though, 22-year-old James Brooks will start on the wing in the place of Oriol Ripol. Brooks came on as a replacement for the Spanish flyer last weekend.
Head coach Wayne Smith said: "We are treating this game in the same way we treated our league game against Leicester at Welford Road. Ulster have never lost a European cup game at Ravenhill, so we have to go out there with the aim to win."
Northampton: Nick Beal; James Brooks, Peter Jorgensen, John Leslie (Capt), Ben Cohen; Paul Grayson, Matt Dawson; Tom Smith, Steve Thompson, Matt Stewart, Steve Williams, Rob Hunter, Grant Seely, Mark Soden
Replacements: 16 Dan Richmond 17 Chris Budgen 18 Robbie Morris 19 Jon Phillips20 Ian Vass 21 Mark Tucker 22 Jon Sleightholme