Liverpool midfielder Joe Allen has been ruled out of the Wales squad for this month's 2014 World Cup qualifiers.
He has been suffering with a shoulder injury and his club manager, Brendan Rodgers, earlier confirmed that Allen needs surgery.
Leeds United striker Steve Morison has also withdrawn because of a calf problem.
Manager Chris Coleman has called in on-loan Millwall striker Jermaine Easter to help cover the withdrawals.
The date of 23-year-old Allen's operation has yet to be confirmed, but having initially been named in the squad, he cannot now face Scotland at Hampden Park on 22 March or Croatia in Swansea on 26 March.
Allen is likely to be out for up to three months when he finally goes under the knife.
Celtic full-back Adam Matthews also misses out with a hamstring injury.
Matthews suffered the problem in the Scottish club's Champions League exit at the hands of Juventus.
Tottenham winger Gareth Bale is included in the squad, with Wales hoping to see a continuation of his stunning recent form which has delivered 11 goals in his last 11 games.
Wales will look to build on an encouraging in February by completing a World Cup double over Scotland on Friday.
That initial victory over the Scots in October was the only win in four World Cup qualifying games so far for Wales, who sit second-bottom of Group A.
It also marked the climax of a tortuous period for new Wales boss Chris Coleman, who had struggled to maintain the improvements made under late boss Gary Speed.
James Collins returns to provide an option at centre-half after missing the 2-1 win over the Austrians.
But Darcy Blake has been deemed short of match fitness and is omitted from the game against Scotland, which marks Gordon Strachan's first competitive game in charge of the Scots.
Uncapped midfielder Jonathan Williams is promoted from the Wales Under-21 squad after scoring in their 3-0 win over Iceland and impressing with his performances for Crystal Palace.
Cardiff City striker Craig Bellamy is again included after pledging to extend his international career.
Arsenal midfielder Aaron Ramsey is also back after being a late withdrawal for the game against Austria.
Wayne Hennessey is a long-term absentee as the Wolves number one continues his rehabilitation from a serious knee injury, so Boaz Myhill, Lewis Price and Owain Fôn Williams are the three goalkeepers selected.
Glyn Myhill (West Bromwich Albion), Lewis Price (Crystal Palace), Owain Fôn Williams (Tranmere Rovers), Ben Davies (Swansea City), James Collins (West Ham United), Chris Gunter (Reading), Joel Lynch (Huddersfield Town), Ashley Richards (Swansea City - loan Crystal Palace), Sam Ricketts (Bolton Wanderers), Ashley Williams (Swansea City), Jack Collison (West Ham United), Andy King (Leicester City), Joe Ledley (Celtic), Aaron Ramsey (Arsenal), David Vaughan (Sunderland), Jonathan Williams (Crystal Palace), Gareth Bale (Tottenham Hotspur), Craig Bellamy (Cardiff City), Jermaine Easter (Crystal Palace - loan Millwall), Simon Church (Reading), Hal Robson-Kanu (Reading), Sam Vokes (Burnley).
Wales Under-21 squad:
Connor Roberts (Cheltenham Town), Danny Ward (Liverpool), Daniel Alfei (Swansea City - loan Wrexham), Lee Evans (Newport County), Kieron Freeman (Derby County), Jonathan Meades (AFC Bournemouth - loan AFC Wimbledon), Scott Tancock (Swansea City), Joe Walsh (Crawley Town), Billy Bodin (Torquay United), Elliott Hewitt (Ipswich Town), Emyr Huws (Manchester City), Lloyd Isgrove (Southampton), Tom Lawrence (Manchester United), Lee Lucas (Swansea City), Wes Burns (Bristol City), Jake Cassidy (Wolverhampton Wanderers), Chris Dawson (Leeds United).