Halkett has been a key player for Hearts this season, playing his best football since moving from Livingston. As well as McKenna, club team-mate John Souttar and Leeds United’s Liam Cooper have also been struggling with injuries.
The 26-year-old gives Clarke options in the centre of defence.
Clark has been called up previously after fine form for St Johnstone and was somewhat a surprise omission from the original squad.
Meanwhile, Stewart’s move to England from Ross County last year has seen him reach new levels. The 25-year-old has 22 goals in League One.
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