Scotland: Ryan Fraser, Matt Ritchie & Barry Douglas withdraw from squad
Bournemouth winger Ryan Fraser is the latest player to drop out of the Scotland squad for the end-of-season friendlies with Peru and Mexico.
Newcastle midfielder Matt Ritchie and Wolves full-back Barry Douglas had already withdrawn.
Hearts centre-back John Souttar is a doubt for the trip, having missed his side's final two Premiership fixtures with a hamstring problem.
Kilmarnock full-back Stephen O'Donnell was called up on Tuesday.
It is a first senior squad inclusion for O'Donnell, 26, who won one Scotland Under-21 cap in 2013.
The former Partick Thistle and Luton right-back helped Killie finish fifth in this season's top flight.
Wolves left-back Douglas, 28, made his international debut in Scotland's friendly win away to Hungary in March.
Ritchie, 28, has a knee problem, while no explanation has yet been given for 24-year-old Fraser's absence.
Seven uncapped players were included in national head coach Alex McLeish's original squad, with the match against Peru kicking off at 02:00 BST on 30 May and the meeting with Mexico at 01:00 on 3 June.
Hearts' Souttar and Jon McLaughlin, Hibernian's Lewis Stevenson and Dylan McGeouch, Aberdeen's Graeme Shinnie, Celtic's Lewis Morgan and Millwall goalkeeper Jordan Archer, as well as O'Donnell, could win their first senior caps.
Celtic quartet Kieran Tierney, Callum McGregor, James Forrest and Leigh Griffiths have been rested, while Liverpool full-back Andy Robertson is preparing for the Champions League final and Fulham pair Tom Cairney and Kevin McDonald are involved in the Championship play-off final.
Revised Scotland squad
Goalkeepers: Jordan Archer (Millwall), Allan McGregor (Hull City) Jon McLaughlin (Hearts)
Defenders: Jack Hendry (Celtic), Scott McKenna (Aberdeen), Charlie Mulgrew (Blackburn Rovers), Stephen O'Donnell (Kilmarnock), Callum Paterson (Cardiff City), John Souttar (Hearts), Lewis Stevenson (Hibernian).
Midfielders: Stuart Armstrong (Celtic), Dylan McGeouch (Hibernian), John McGinn (Hibernian), Kenny McLean (Aberdeen), Scott McTominay (Manchester United), Jamie Murphy (Brighton & Hove Albion, on loan at Rangers), Graeme Shinnie (Aberdeen), Ryan Christie (Celtic, on loan at Aberdeen).
Forwards: Oli McBurnie (Swansea City, on loan at Barnsley), Lewis Morgan (Celtic, on loan at St Mirren), Matt Phillips (West Bromwich Albion), Johnny Russell (Sporting Kansas City).