Derby County striker Jamie Ward has become the latest player to withdraw from the Northern Ireland squad for the friendlies in Uruguay and Chile.
Ward, 28, has been named in Derby's squad for Saturday's Championship play-off final against Queen's Park Rangers.
According to the Irish FA, he felt it was not in his best interests, or those of the national team, to go to South America after an extended club season.
Fleetwood midfielder Conor McLaughlin has been called into the squad.
McLaughlin will join up with manager Michael O'Neill's party after Fleetwood's League Two play-off final against Burton Albion on Monday.
Northern Ireland play Uruguay in Montevideo on Friday 30 May and then take on Chile in Valparaiso on Wednesday 4 June.
Ward's withdrawal is another blow to O'Neill, who had expressed his frustration at losing players.
On Tuesday, Huddersfield Town striker Martin Paterson opted out of the squad following a series of injuries this season.
Other players not making the trip include defenders Jonny Evans (Manchester United), Gareth McAuley (West Bromwich Albion), Ryan McGivern (Hibernian), and Lee Hodson (Milton Keynes Dons).
Alex Bruce, Craig Cathcart, Rory McArdle, Alan Mannus, Chris Brunt, Pat McCourt, Kyle Lafferty and Will Grigg are also absent.
Northern Ireland squad
Goalkeepers: Roy Carroll (Olympiakos), Michael McGovern (Falkirk), Trevor Carson (Portsmouth).
Defenders: Daniel Lafferty (Burnley), Ryan McLaughlin (Liverpool), Aaron Hughes (QPR), Luke McCullough (Doncaster), Liam Donnelly (Fulham), Chris Baird (Burnley).
Midfielders: Steven Davis (Southampton), Oliver Norwood (Huddersfield Town), Shane Ferguson (Newcastle Utd), Corry Evans (Blackburn Rovers), Sammy Clingan (Kilmarnock), Jonny Steele (New York Red Bulls), Paul Paton (Dundee Utd), Conor McLaughlin (Fleetwood Town).
Forwards: Niall McGinn (Aberdeen), Billy McKay (Inverness CT), Dean Shiels (Rangers), Josh Magennis (Aberdeen).