Captain and Manchester City striker Bellamy is set to miss the rest of the season because of a knee injury.
And Toshack was dealt a further blow when Fulham midfielder Davies was ruled out for the rest of the season after having surgery on a foot problem.
So Allen, King and Reading striker Simon Church, who scored four goals on loan at Leyton Orient this season, are set to be handed their international chance.
They hope to follow Arsenal's Aaron Ramsey and West Ham's Jack Collison on to the biggest stage.
Collison is on the road to full recovery after suffering a knee injury in a 1-0 win at Wigan Athletic in March and Toshack hopes he will be available for Wales' summer fixtures.
So Allen and King are set to make the step up from the Wales Under-21 team and win their first full Wales cap against Estonia.
Should they perform they could book a seat on the plane to Azerbaijan.
King, who has scored once in six Wales Under-21 appearances, has been one of the stand-out performers for Leicester as they won promotion back into the Championship and he won the club's young player of the season award.
The 11-goal midfielder - who has attracted attention from Premier League sides Aston Villa, Middlesbrough and new boys Wolverhampton Wanderers - was also nominated for the players' player of the season and player of the season awards.
Allen spent part of the season on loan at non-league Wrexham but after Ferrie Bodde's season was curtailed prematurely because of a knee injury in November, Allen was given his chance by Swans boss Roberto Martinez.
The performances of Allen - who has scored once in 11 Wales Under-21 caps - have been hailed by manager and team-mates and now his international dreams are set to be realised.
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