Dutch forward Anduele Pryor and Manchester United youngster Anton Bryan both spent time with the Robins last week.
Johnson has already decided, though, not to offer Bryan a contract, but is yet to make his mind up on former Vitesse Arnhem hitman Pryor.
"Anton Bryan was a good player but nothing more than we already had in the squad, especially with a few younger lads coming through in that position," he told his club's official website.
"Anduele Pryor had a fantastic campaign two or three seasons back but has lost his way a little bit since then. We're still having a look at him but we need to make a decision soon."
Source: Team_Talk