Palace spent £20million on Henderson last summer but Johnstone was first-choice until he suffered a calf injury in December.
Henderson has impressed in his place since then and is expected to come back into the team when Palace travel to Arsenal in the Premier League on Saturday.
“Sam Johnstone will play tomorrow”, said Palace boss Hodgson. “Dean was the No1 in the last league game, in the last games we’ve played, and has conducted himself well.
“Sam has his chance tomorrow, and maybe Dean will come back into the game against Arsenal. More than likely, that will happen.
“Then we’ll move on to the next game and then with both of them knocking on my door saying, ‘I’m the No1, I’m the best goalkeeper’, I will have to make a decision which one to play.”
Johnstone has established himself as second-choice goalkeeper for England behind Jordan Pickford.
But he and Henderson may be competing for a spot in Gareth Southgate’s squad for Euro 2024.
“Sam came into the team when Vicente Guaita got injured”, said Hodgson. “He kept his place, which relegated Guaita to the second spot which he didn’t accept, so we ended up losing him — unfortunate because he was a good goalkeeper.
“The club found Dean Henderson, and he had to wait. He got injured on [his debut], so that wait was partly in rehabilitation. But he would probably have had to wait even if he hadn’t been injured, because Johnstone had the shirt at the time.
“Now Dean Henderson got into the team again because of Johnstone’s injury.”
Hodgson has admitted that Palace could have difficulty convincing Henderson or Johnstone to stay at Selhurst Park beyond the summer if they are not playing regularly.
“It’s not easy to have two goalkeepers of that quality,” he said. “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.”