A "diagonal ball" refers to a pass or kick in a sport, like soccer or football, where the ball is sent at an angle, not directly straight. It is usually used to reach a teammate who is positioned away from the passer in a diagonal direction.
When the phase is begun following a long diagonal ball from midfield or defence. (kopleft.com)
They took up positions between the outer centre - backs and the wing - backs and closed in from the outside, preventing the short diagonal ball out wide. (footballwhispers.com)
You don't always need a midfielder who will always play long diagonal balls. (justarsenal.com)