The Ospreys face Leicester Tigers at Liberty Stadium on Sunday knowing they must win in order to keep their Heineken Cup quarter-final hopes alive.
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Tigers lead Pool Two and victory in Wales will leave the two-time champions in pole position ahead of next weekend's showdown with Toulouse.
Reigning Pro12 champions Ospreys are third in Pool Two and currently stand four points behind Toulouse.
Ospreys have lost just one of their last four Heineken Cup matches against Leicester but four of the seven meetings between the two have gone the Tigers' way.
"This is as big as it comes. It's a massive game and is one that we can't afford any slip-ups in," said Ospreys head coach Steve Tandy.
"We've done well over the last few weeks to put ourselves in contention in the PRO12 and in Europe, but we realise that we'll need to go up another level this weekend in terms of performance if we want to get the result we need to stay in the competition.
"They'll come down here confident after some good results of their own lately and we know that we are going to have to match them physically. They've got the advantage in the pool as it stands but there's everything to play for."
In team news, Ospreys have confirmed that Ashley Beck will be sidelined for both Sunday's match and the final group game against Benetton Treviso because of an ankle ligament injury - as a result Tom Isaacs and Andrew Bishop line up in the centres.
Meanwhile, Tom Youngs, Dan Cole and Geoff Parling are amongst the senior internationals recalled to the Leicester Tigers team.
Forward trio Cole, Youngs and Parling, plus centre Manu Tuilagi, all return in changes to the team that started the Premiership win over Worcester Warriors last week, while Anthony Allen, Marcos Ayerza, Julian Salvi and Jordan Crane also get the nod to start.
Ospreys: 15 Richard Fussell, 14 Ross Jones, 13 Tom Isaacs, 12 Andrew Bishop, 11 Eli Walker, 10 Dan Biggar, 9 Kahn Fotuali'i (c), 8 Joe Bearman, 7 Justin Tipuric, 6 Ryan Jones, 5 James King, 4 Ian Gough, 3 Adam Jones, 2 Richard Hibbard, 1 Ryan Bevington
Replacements: 16 Scott Baldwin, 17 Duncan Jones, 18 Campbell Johnstone, 19 Lloyd Peers, 20 Sam Lewis/Jonathan Thomas, 21 Rhys Webb, 22 Matthew Morgan, 23 Jonathan Spratt
Leicester Tigers: 15 Mathew Tait, 14 Niall Morris, 13 Manusamoa Tuilagi, 12 Anthony Allen, 11 Adam Thompstone, 10 Toby Flood, 9 Ben Youngs, 8 Jordan Crane, 7 Julian Salvi, 6 Steve Mafi, 5 Geoff Parling, 4 Louis Deacon (c), 3 Dan Cole, 2 Tom Youngs, 1 Marcos Ayerza
Replacements: 16 Rob Hawkins, 17 Logovi'i Mulipola, 18 Martin Castrogiovanni, 19 Ed Slater, 20 Thomas Waldrom, 21 Sam Harrison, 22 George Ford, 23 Matt Smith