Johnson was not in the squad at QPR on Saturday because of his mother's illness.
Gordon Strachan: 'He trained today and he's had a difficult week.
'We kept in contact with him all the time because it's not been a great week for him and his family. We're relieved now things are getting better for the family. We said, "Come back when you're ready."
'We needed to win on Saturday but the most important thing was his family.'