Ospreys require a victory to give themselves any chance of acquiring a Champions Cup spot for next season, even then it may still not be enough.
Head coach Steve Tandy said this week: “There’s a really positive mood in the camp this week. We realise the size of the challenge ahead, going up against a very good Ulster side who need the win for a play-off spot but we know that if we can go again and deliver another positive performance that we could get our own reward and it’s really exciting. Everybody is looking forward to Saturday afternoon and getting stuck into the job.
Rhys Webb will captain the Welsh region.
Meanwhile, Craig Gilroy, a late withdrawal from the team last week, is reinstated to the starting Ulster XV, which is otherwise unchanged from the 30-6 win over Leinster.
Rory Best will again captain the side, with props Callum Black and Ricky Lutton, and second rows Pete Browne and Franco van der Merwe joining him in the front five. The back row trio of Iain Henderson, Chris Henry and Sean Reidy, which performed so effectively last week, is also retained.
Man of the Match in that game, Ruan Pienaar, will line out alongside Paddy Jackson at half-back, with Stuart McCloskey and Luke Marshall continuing their midfield partnership. Gilroy, Andrew Trimble and Jared Payne complete the backline.
The bench, which helped to deliver a strong second half performance last weekend, is also unchanged. Rob Herring, Kyle McCall, Andy Warwick, Robbie Diack and Roger Wilson cover the forward positions, while Paul Marshall, Stuart Olding and Darren Cave are included as backline options.
Form: The Welshmen have won their last three matches, all with maximum points, since their home defeat to Scarlets at the end of March. A maximum point win would guarantee Ulster a Play-Off spot, and also give them any outside chance of that game being in Belfast. Anything less and they will be looking at other results to confirm their place in the last four for a fourth successive year. The Ulstermen's only defeat in the last five rounds of Guinness PRO12 play was at Glasgow in round 18. Ulster have lost just one of their last five fixtures against Ospreys: 20-31 at Liberty Stadium in the equivalent fixture in round 10 last season.
Ospreys: 15 Dan Evans, 14 Ben John, 13 Owen Watkin, 12 Josh Matavesi, 11 Eli Walker, 10 Dan Biggar 9 Rhys Webb (c), 8 James King, 7 Sam Underhill, 6 Dan Lydiate, 5 Rory Thornton, 4 Rynier Bernardo, 3 Dmitri Arhip, 2 Scott Baldwin, 1 Nicky Smith
Replacements: 16 Scott Otten, 17 Gareth Thomas, 18 Aaron Jarvis, 19 Joe Bearman, 20 Olly Cracknell, 21 Brendon Leonard, 22 Sam Davies, 23 Tom Grabham
Ulster: 15 Jared Payne, 14 Andrew Trimble, 13 Luke Marshall, 12 Stuart McCloskey, 11 Craig Gilroy, 10 Paddy Jackson, 9 Ruan Pienaar, 8 Sean Reidy, 7 Chris Henry, 6 Iain Henderson, 5 Franco van der Merwe, 4 Pete Browne, 3 Ricky Lutton, 2 Rory Best (c), 1 Callum Black
Replacements: 16 Rob Herring, 17 Kyle McCall, 18 Andrew Warwick, 19 Robbie Diack, 20 Roger Wilson, 21 Paul Marshall, 22 Stuart Olding, 23 Darren Cave
Date: Saturday, May 7
Kick-off: 15:00 BST (14:00 GMT)
Venue: Liberty Stadium
Referee: Marius Mitrea (Italy)
Assistant Referees: TBC