Charles Piutau and Bundee Aki have been ruled out of the Barbarians' clash with England at Twickenham on Sunday, live on Sky Sports.
New Zealand international Piutau suffered an Achilles strain in the warm-up Ulster's European Champions Cup play-off victory over the Ospreys on Saturday.
Ireland centre Aki, who was due to captain the Baa-Baas against England, is nursing an ankle injury.
The Barbarians also announced that Sale Sharks fly-half AJ MacGinty has been withdrawn from the squad by his club.
"It's unfortunate that Charles Piutau strained an Achilles tendon warming up for Ulster on the weekend," Babarians head coach Pat Lam said.
"Bundee Aki rolled an ankle in training in Galway and AJ MacGinty has been pulled out by his club."
Lam has now finalised his group for the England encounter, adding five more players in Scotland fly-half Finn Russell, New Zealand centre Malakai Fekitoa, Newcastle and Fiji centre Josh Matavesi, his Falcons colleague - flanker Nili Latu - and Connacht prop Denis Buckley.
"It's great that we are able to call in some more quality players following the weekend's semi-finals," Lam continued.
"Finn Russell comes in from Glasgow, Malakai Fekitoa from Toulon and Josh Matavesi from Newcastle, and they will all add to the Barbarians way."
"I've worked with Denis Buckley at Connacht, and he is a powerful scrummager with a high work-rate around the pitch, while Nili Latu has just finished a superb spell at Newcastle and his destructive power and experience across the back-row will be a great asset for us."
The Barbarians starting line-up is due to be announced on Thursday.
Barbarians squad:
Backs: Niyi Adeolokun (Connacht & Ireland), Chris Ashton (Toulon & England), Malakai Fekitoa (Toulon & New Zealand), Greig Laidlaw (Clermont & Scotland), Luke McAlister (Toulon & New Zealand), Josh Matavesi (Newcastle Falcons & Fiji), Semi Radradra (Toulon), Finn Russell (Glasgow Warriors & Scotland), Josua Tuisova (Toulon & Fiji), Rhodri Williams (Bristol & Wales).
Forwards: John Afoa (Gloucester & New Zealand), Denis Buckley (Connacht), Ultan Dillane (Connacht & Ireland), Juan Martin Fernandez Lobbe (Toulon & Argentina), Ramiro Herrera (Stade Francais & Argentina), Benjamin Kayser (Clermont & France), Nili Latu (Newcastle Falcons & Tonga), Tatafu Polota-Nau (Leicester & Australia), Sateleki Timani (Clermont & Australia), Justin Tipuric (Ospreys & Wales), Loni Uhila (Clermont), Flip van der Merwe (Clermont & South Africa), Victor Vito (La Rochelle & New Zealand).