Henderson, who joined last summer for £20million, is set to get the nod against Arsenal on Saturday, having done well since he replaced Johnstone after he suffered a calf injury last month.
Palace boss Roy Hodgson has suggested the No1 spot is Henderson’s to lose, but Johnstone is back fit again and did little wrong until his injury.
As well as starting for Palace, Henderson and Johnstone need to be playing regularly to boost their chances of making the England squad for Euro 2024.
It is an unusual situation and a tricky one to manage for Hodgson, who is still under the spotlight as the Eagles continue to struggle.
Palace have managed just one win in 12 games since the end of October and are nervously looking over their shoulders.
They are five points above the bottom three, and Hodgson faced chants of “you don’t know what you’re doing” after he substituted Eberechi Eze with Palace chasing an equaliser in their 1-0 FA Cup defeat at Everton on Wednesday.
If Henderson does start in goal on Saturday, he will expect a busy afternoon against an Arsenal side looking to make a statement after a blip before their two-week winter break. With Johnstone waiting in the wings, the focus will be on him.
“It’s not easy to have two goalkeepers of that quality,” Hodgson said earlier this week as he admitted the difficulty in keeping both 'keepers at Selhurst Park beyond the summer.
“It will be a situation the club will have to look closely at. Whether they will be able to persuade both that, ‘We need you both’.
"Or whether they will be forced to accept it’s probably a bit ambitious to have two England goalkeepers where one is not going to play each week, I’m prepared just to let the time go.
“When that time comes, the club will make a decision. It’s much more [sporting director] Dougie Freedman and [chairman] Steve Parish who will be confronted with that.”