The only sure way to get a Diamond Player is to buy one at auction. My tip is simple, once you have exhausted MyTeam mode, if you still do not have a Diamond player (highly likely), sell off all extra assets.
I was able to net around 100,000 currency having about 300 cards. The cheapest Diamond at the time (Isiah Thomas, Celtics, Playoff Edition) was only 46,500.
Also pro-tip, try auctioning any golds a little over what you could get from the CPU.
Can honestly say I bought over 70 packs and no Diamond, save yourself the heartache and go auction. Good luck!