For their fourth Pool C game, the Ravens management, headed by Gary Longwell, have made seven changes in personnel.
Full-back Mark McCrea, centre Michael Allen and scrum half Michael Heaney have been drafted into the back-line, with Jamie Smith, Jonny Shiels and Ian Porter making way.
In the front row, Andi Kyriacou and Jerry Cronin's involvement in the Heineken Cup squad sees Niall Annett and Alan Whitten selected as starters.
Ian Henderson gets a call-up in the second row, where he partners Ryan Caldwell, and USA international Scott LaValla will line out at blindside flanker, with David McGuigan and Neil McComb completing the back row.
After losing to the Bedford Blues and Neath, the third-placed Ravens need to win on Saturday afternoon to keep in contention for a quarter-final berth.
They also need Bedford or Moseley to slip up in their remaining games, and Neath are snapping at their heels too.
Saturday's opponents, London Welsh, are coached by former England hooker Phil Greening and have lost their first two games in this season's competition.
The sides met in the 2009/10 British & Irish Cup, at Ravenhill last February. On the night, tries from Paul Marshall and Timoci Nagusa, allied to 11 points from Niall O'Connor's boot, helped the Ravens to a 21-17 win.
ULSTER RAVENS (v London Welsh): Mark McCrea; Tommy Seymour, Michael Allen, Luke Marshall, Craig Gilroy; Paddy Jackson, Michael Heaney; Conor Carey, Niall Annett, Alan Whitten, Ryan Caldwell, Ian Henderson, Scott LaValla, David McGuigan, Neil McComb.
Replacements: John Burns, Kyle McCall, Ross Hackney, Paul Pritchard, Blane McIlroy, Niall O'Connor, Jonny Shiels.