First I would adjust formation so that that it could require less key players position that reduce current problem a bit. Then, next step would be filling key player's position by academy players and since I have to chose by one day , then definitely I would go for that player that has confidence, skills, hardworking and overall good attributes that major squad's players can rely on. That one day, I would give him a important drills , techniques and hold a discussion with senior teams so that they can better understand each other.