Here is my take on the subject...
HCG when injected introduces LH to your system. LH in turn triggers the body to produce test. If you inject HCG (LH) than it stands to reason your body will see this and continue to supress natural production of LH. For how long, I can find little information online. It stands to reason that introducing anything into your system that will cause your body to continue to supress its natural production, through one path or another, should be avoided post cycle. For this reason I always suggest only using HCG during cycle. Very near the end of the cycle can be a great help in getting a "kick start" for the testes in terms of size / volume. Hopefully this helps??
Take a look at the short descriptions I have pasted below for HCG and the male hormonal axis. They may help give insight into how both work and would affect one another.
Here is a short description of HCG:
Human chorionic Gonadotropin (HCG) is a dimer glycoprotein which consists of a 237-amino-acid (aa) sequence. HCG is composed of two nonidentical subunits. The alpha subunit consists of a 92-aa sequence identical and shared in common with the pituitary glycoprotein hormone FSH, LH, TSH. The beta subunit, the front 115-aa piece is the same as beta-LH subunit, the last 30-aa tailpiece is not present in beta-LH, differs just enough to conform specific biologic activity on the intact hormones. HCG is secreted by placenta. There are two types of cells in the placenta. The cytotrophblastic cells secrete dimer hCG and the syncytial trophoblast cells secrete free alpha-hCG. There is no free beta-hCG in the placenta, but it is in blood.
The above was taken from:
http://www.science.mcmaster.ca/Biology/4S03/HCG.html
Here is the short / simple description of the male hormonal axis:
Every day, a man's testes are busy producing millions of sperm. As in the female cycle, the hypothalamus starts the process by secreting gonadotropin-releasing hormone (GnRH) into the bloodstream. GnRH is released approximately every 90 minutes, triggering the pituitary gland to release both luteinizing hormone (LH) and follicle stimulating hormone (FSH) into the bloodstream, which carries the hormones to the testes. Within the testes, LH goes directly to the Leydig cells, which triggers the production of testosterone; FSH goes directly to the Sertoli cells, which triggers the production of sperm. When testosterone levels drop, this tells the hypothalamus to release more GnRH, which tells the pituitary gland to secrete more LH and FSH, which tells the body to make more testosterone and sperm. As we discuss later in this article, this hormonal interrelationship, clinically called the hormonal axis, must be in tip-top shape in order for men to produce enough high-quality sperm to fertilize an egg. Trouble with either the Leydig or Sertoli cells may cause unusually high levels of either LH or FSH or unusually low levels of testosterone. Trouble in the hypothalamus or pituitary glands will cause a breakdown of the entire hormonal axis. In all cases, sperm counts will be low or even nonexistent.
The above was taken from:
http://my.webmd.com/content/article/1680.51511