Byrne damaged ligaments in his left foot during the Six Nations clash between Wales and Ireland and has not played since. He hopes to be available to play again within a few weeks.
Also missing from the Ospreys line-up is another of their British & Irish Lions contenders, Gavin Henson. He is now two weeks into his recuperation after twisting his ankle in the EDF Energy Cup semi-final defeat to Gloucester.
That caused damage to his ligaments on both the lateral and medial sides of his right ankle and he is likely to be sidelined for at least another month.
Both Byrne and Henson were seen as front runners for inclusion in Ian McGeechan's squad for South Africa this summer. The Lions tour party is due to be announced on 21 April.
McGeechan and his Lions coaching staff will be at all the Heineken Cup quarter-finals this weekend having one final look at much of the best available playing talent.
Here are the teams for the four games:
Cardiff Blues v Toulouse
Millennium Stadium, Cardiff - Saturday 11th April 2009, 15:30
Cardiff Blues: B Blair; L Halfpenny, T Shanklin, J Roberts, T James; N Robinson, J Spice; G Jenkins, G Williams, F Filise, B Davies, P Tito [capt], M Molitika, M Williams, X Rush.
Replacements: J Yapp, T Thomas, D Jones, A Powell, R Rees, C Sweeney, G Thomas
Toulouse: M Medard; V Clerc, F Fritz, Y Jauzion, C Heymans; F Michalak, B Kelleher; D Human, W Servat, S Perugini, F Pelous, P Albacete, J Bouilhou [capt], T Dusautoir, S Sowerby.
Replacements: A Vernet Basualdo, Y Montes, G Lamboley, Y Nyanga, D Skrela, M Kunavore, C Poitrenaud
Referee: Chris White (England)
Leicester Tigers v Bath Rugby
Walkers Stadium, Leicester - Saturday 11th April 2009, 18:00
Leicester Tigers: G Murphy [capt]; S Hamilton, D Hipkiss, S Vesty, A Tuilagi; T Flood, H Ellis; M Ayerza, G Chuter, M Castrogiovanni, L Deacon, B Kay, T Croft, B Woods, B Pienaar.
Replacements: M Davies, D Cole, J Crane, C Newby, J Dupuy, A Erinle, J Murphy
Bath Rugby: N Abendanon; J Maddock, A Crockett, S Hape, M Banahan; B James, M Claassens; D Flatman, L Mears, D Bell, J Harrison, P Short, A Beattie, M Lipman [capt], J Fa'amatuainu.
Replacements: R Hawkins, P Ion, S Hooper, J Scaysbrook, S Bemand, S Berne, A Higgins
Referee: Alan Lewis (Ireland)
Munster v Ospreys
Thomond Park, Limerick - Sunday 12th April 2009, 13:00
Munster: P Warwick; D Howlett, K Earls, L Mafi, I Dowling; R O'Gara, T O'Leary; M Horan, J Flannery, J Hayes, D O'Callaghan, P O'Connell [capt], A Quinlan, D Wallace, D Leamy.
Replacements: D Fogarty, T Buckley, M O'Driscoll, N Ronan, P Stringer, B Murphy, D Hurley
Ospreys: T Bowe; J Vaughton, S Parker, A Bishop, S Williams; J Hook, M Phillips, P James, H Bennett, A Jones, I Gough, A Jones, R Jones [capt], M Holah, F Tiatia.
Replacements: R Hibbard, C Griffiths, F Levi, A Lloyd, T Smith, J Nutbrown, D Biggar
Referee: Wayne Barnes (England)
Harlequins v Leinster
The Twickenham Stoop, Twickenham - Sunday 12th April 2009, 15:30
Harlequins: M Brown; D Strettle, G Tiesi, J Turner-Hall, U Monye; N Evans, D Care; C Jones, G Botha, M Ross, J Percival, G Robson, C Robshaw, W Skinner [capt], N Easter.
Replacements: T Fuga, M Lambert, J Evans, T Guest, A Gomarsall, C Malone, T Williams
Leinster: R Kearney; I Nacewa, B O'Driscoll, G D'Arcy, L Fitzgerald; F Contepomi, C Whitaker; C Healy, B Jackman, S Wright, L Cullen [capt], M O'Kelly, R Elsom, S Jennings, J Heaslip.
Replacements: J Fogarty, R McCormack, T Hogan, S O'Brien, S Keogh, G Dempsey, S Horgan
* Girvan Dempsey has been named at No 21 on the replacements bench pending the outcome of Jonathan Sexton's appeal hearing later today. If the appeal is successful Sexton will replace Dempsey at No 21.
Referee: Nigel Owens (Wales)