Both players are out of contract this summer.
Marshall was a high-profile free transfer signing from Bradford City at the start of the 2017-18 campaign but the winger, who turns 32 next month, has struggled to hold down a starting berth since Bowyer replaced Karl Robinson in the SE7 hotseat.
Fosu has struggled to find form this season and has not featured since the end of March.
“Everybody is fit and we can only pick 18 players,” said Bowyer. “At the moment the 18 I’m picking deserve to be in the 18, nothing more than that.
“I just go on how people are training and playing. That’s the reason they are not in it.
“It’s competition for places. That is not just our football club, that’s every football club. The people who are training and playing the best are the ones involved. It is nothing personal.”
Bowyer is expecting a clean bill of health for Good Friday’s match at Oxford United. Igor Vetokele came off with a tight hamstring while Krystian Bielik was also subbed suffering with cramp.
Oxford are fourth in the form guide over the past 10 matches – taking 20 points to pull away from drop danger in League One.
Charlton are three points behind second-placed Barnsley with four fixtures to play. Sunderland and Portsmouth, both above the South Londoners, have an extra match in hand.
The maximum the Addicks can finish on is 91 points.
Bowyer does not lay down mini targets from sections of the fixture list. He said: “I set mine at the start of the season – that if we got 92 points it guarantees you top two.
“If we do win all of our last four we are not far off it.
“We’ll go there on Friday and we’ll try our hardest to win the game.
“Nothing changes for us. We respect everybody that we play. Every team has got strengths and weaknesses – that includes us.”