The book on Foster was finally revealed by Tom Nalen on ESPN radio. He apparently had a wrist injury that his agent told him to keep quiet about. It got around, but not to Shanny for some reason and he was drafted where he was - probably out of panic. Nails was saying his hand never got better and he was surprised he played as long as he did.
I remember seeing how horrid he was every week and I got so tired of it I made a signature that said "Get Foster off the roster!" and featured Foster's face being flushed down a toilet. I don't remember why, but Taco really liked Foster for some reason (it wasn't his play) and so he retaliated and made a similar signature that said "Get Khan off the John." Foster was out of football two years later.
The only reason he played here as long as he did was because Shanahan was covering up what a bad pick he was. That always made me really mad. It's one thing to be a bad GM, but if a pick busts, move on. Don't let him start ahead of better players because of your ego. If Shanahan hadn't drafted him but was given him by another GM, he wouldn't have ever played him.