It may not seem very realist but it does happen, teams can have injury/suspension crisis and yes it can happen to players that play in the same positions, sometimes that might be down to how you play or how your opponents play, If Arsenal had Giroud and Walcott injured in the same week, they would be struggling to field a striker, Gervinho is at the African cup of nations and Chamakh and Bendtner are out on loan to other clubs.