Chris Weale started last week against Derby after Adriano Basso was injured against Blackpool and last season's player of the year faces a tough battle if he wishes to get back into the team.
Johnson refused to be drawn on the goalkeeper situation saying: "It's a week by week decision for me at the moment. Both keepers are looking sharp.
"Stephen Henderson got 45 minutes for the Republic of Ireland U21's in midweek, so they're all training like they are competing for a first team place," he added.