Because having the blood test already, the doctor is going to already have a basis to know that your free testosterone in the saliva test is going to be some approximate amount that is reasonable.
Unless you want to do something fairly pointless like make a 90/10 mixture that is mostly your stuff, you are going to wind up making something that is an obvious fraud.
A salivary test is NOT going to reveal a substantially lower free testosterone number than a recently done blood test. Especially not after just getting an injection of testosterone.
He's probably been around the block a few times and knows that after an injection such as he just gave you, from your starting point your results is going to be between here and there.
The whole thing is just assuming he's a complete fool.
And no he doesn't need a fraudulent salivary test to be able to continue providing you with testosterone and HCG.
Also with regard to estrogen, depending on the particular woman and time of the month her estrogen levels might be 6 or 7 times beyond the normal range for men. So if you mix much of hers in there, now (if we were to assume he didn't realize you were treating him as a fool) what is that supposed to do with his efforts to help you medically?
You've found someone that wants to help you and you want to just go completely off the wall in how to fake results. It's up to you, go ahead.
But let's say that you've decided that actually this was a bad idea.
If I understand correctly and you mean he gave you the injection to take home, if you haven't taken it yet, then why not take only half of it prior to the salivary test, and do that test the next day. If he expresses surprise that the increase was so low, you could ask if he thinks the shot had time to take effect yet: you did the salivary test the next day. If he knows about the pharmacokinetics of it he would understand that would result in less effect.
Or if you're convinced your testosterone is naturally nowhere near as bad now and you haven't taken the shot, then why not just do the salivary test without the shot.