Billy King's late call up for Thursday's friendly in Hungary takes the number of Hearts players in the Scotland Under-21 squad up to five.
Kilmarnock's Mark O'Hara and Aberdeen's Cammy Smith have also been drafted in by caretaker manager Danny Lennon.
The trio replace Killie's Craig Slater, Jason Cummings of Hibernian and Ross County's Marcus Fraser, who have withdrawn through injury.
Hearts wrapped up the Championship title at the weekend.
Callum Paterson, Sam Nicholson, Jack Hamilton and Jordan McGhee are the other Tynecastle players included by former St Mirren boss Lennon.
"I got a text from my mum saying congratulations," said Paterson, who was called into Gordon Strachan's Scotland squad for the games against the Republic of Ireland and England in November.
"That was the end of my celebrations.
"Winning the league is in the back of my mind.
"I am here to focus on playing for the Scotland Under-21s. It is one step away from the A squad and that is where you aspire to play.
"I was lucky enough to be picked for the squad when I was in the Championship so if I improve myself and do well at my club then hopefully I can do well again."
Scotland Under-21 squad
Goalkeepers: Jack Hamilton (Hearts). Jonathan Henly (Reading)
Defenders: Stuart Findlay (Celtic), Mark O'Hara (Kilmarnock), Dominic Hyam (Reading), Stephen Kingsley (Swansea City), Donald Love (Manchester United), Jordan McGhee (Hearts), Callum Paterson (Hearts)
Midfielders: Ryan Christie (Inverness Caledonian Thistle), Daniel Handling (Hibernian), Callum McFadzean (Sheffield United), John McGinn (St Mirren), Sam Nicholson (Hearts), Charlie Telfer (Dundee United), Cammy Smith (Aberdeen)
Forwards: Ryan Fraser (Bournemouth), Ryan Gauld (Sporting Clube de Portugal), Declan McManus (Aberdeen), Lawrence Shankland (Aberdeen), Billy King (Hearts)