Johnstone, 18 and on-loan at the Iron from Manchester United, has missed the last two games after dislocating the little finger in his right hand.
Sam Slocombe has deputised admirably, keeping a clean sheet in the goalless draw at MK Dons on Saturday and taking man-of-the-match honours in Tuesday's 2-2 stalemate with Huddersfield at Glanford Park.
If Johnstone is fit he is likely to come back into the starting line-up, with Slocombe dropping back to the bench.
Jordan Robertson return to training this week for the first time since breaking an ankle in August, but is not included in the squad.
Cliff Byrne (back) will also not feature despite being back in training for a couple of weeks, while Niall Canavan (ankle) is also unlikely to be involved despite returning to training.
Midfielder Damien Mozika remains out (ankle), but is not far from a return having sat out the last two games.
Josh Lillis, Jimmy Ryan (both knee) and Matt Godden (hip/hernia) are long-term absentees.
Tranmere are hoping to welcome back forward Enoch Showumni, after an ankle injury kept him out of Tuesday night's win at Hartlepool.