Defending is not only the responsibility of defenders in today’s modern game.
The golden rule is:
When losing possession of the ball, the entire team should get behind the ball and try to win it back. Therefore, attacking players share defensive responsibilities.
These responsibilities depend on the team’s own system of play, the coach’s philosophy and the types of players available. There are top strikers that will not share defensive responsibilities based on their own will or based on a coach’s philosophy.
There is something to consider: A top striker that guarantees 20-30 goals per season might lose his effectiveness if he has too many defensive responsibilities. It does not mean that he has no defensive responsibilities. However, it is certainly more intense for a lone striker to defend than two strikers.
As stated before, the team’s own system of play and the team’s strategy must be considered as well. A system of play can work in favour of certain players or actually make their life harder at times. The opponent’s system of play is also of high importance in a game.