Johnston made his Celtic debut in May 2017 alongside Anthony Ralston against St Johnstone, two years later he was in the side that started the Treble Treble clinching Scottish Cup Final win over Hearts.
Injuries and changes of managers made progress difficult for Johnston which was best summed up by the reaction of Neil Lennon to an injury picked up away to St Johnstone in January 2020.
Loan spells at Vitoria Guimareas and West Brom were part shop window and part fitness exercises with Johnston able to get the regular involvement that was always elusive at Celtic.
Financial issues have ruled out a permanent move to West Brom but it seems that Sheffield United see Johnston as being able to contribute the goals needed to fire up their promotion push.
Celtic could be looking at an upfront fee close to £5m with add ons built in with promotion unlocking over £100m if Sheffield United go up with Johnston involved.
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