First thing here is try to remember that a good manager always finds a way to make room for someone that will improve his performance and make his life better. No one will throw away an opportunity if they can see and feel it clearly! But how do you create the opportunity when the manager doesn't really believe you have such magic!
“I see, does that mean you have all the people you need to perform your position or do you feel you could improve your performance with a better mix or an addition but there is no money to hire?” If the answer is “I have all the people I need”, you have a tougher position. But the general strategy is to try to get a “yes”. According to the basic universal law, there is always room for improvement; “It would be nice though if you had a contingency backup, wouldn't it?, I see, but you do agree that things can change or there is always room to improve things, isn't there?”
Another slightly different approach which is a bit more aggressive is to say “In the event that we can come across someone that is a fit for your area and is head & shoulders over the crowd, when should I call you back?” Another question, “Have you ever made room for an outstanding person?”
If there is a budget problem, “Barring budget restrictions, do you need to add people?” “When does your new budget begin?” Some way out questions if you have the guts to use them, “Doesn't the pressure of having to hire sometimes blur the decision process into taking what's available?, Don't you think it would be better to have several people in mind, to act on the opening as soon as it occurs?”