The 18-year-old, on loan from Manchester United, missed Saturday's goalless draw at MK Dons but returned to full training on Monday.
Although Sam Slocombe kept a clean sheet in his absence, the Iron manager Alan Knill is likely to revert to Johnstone between the posts.
Damien Mozika, the midfielder, remains absent with an ankle injury.
Cliff Byrne (back) and Michael Collins (tendonitis) are in full training but, having been left out of Saturday's squad due to a lack of match fitness are again unlikely to make the matchday 16.
Matt Godden (hip/hernia), Jordan Robertson (ankle) and Jimmy Ryan (knee) remain long-term absentees.