After doing a little homework, I have concluded that Millard is destined to fill in where Delanie left off as a jack-of-all-trades Swiss Army Knife, playing the role of TE2, FB, short yardage power back, and special teams ace (although he won't return kicks) for years to come when he recovers from his ACL, be that late 2014 or 2015.
This is not to say that Bruce Miller will be displaced per say, as I think Bruce will be hard to beat out straight up as a fullback on obvious running downs. But Trey has an exceptional amount of athleticism and versitility that Bruce simply cannot match.
At Oklahoma, Trey showed an ability to be a good route runner with good hands and a great ability to provide YAC, be it with power through contact or agility and surprising quickness (many times both). He will only get better here in San Francisco.
He is also an absolutely devistating blocker. He is about as powerful as they come at his size and has the speed and athleticism to get to where he needs to be as well as the high football IQ to recognize his role in a given situation (granted I didn't watch years worth of game tape and I'd have zero way of knowing if he did do something wrong unless it was extremely obvious, so I'm taking the word of scouting reports done by people who know much more than me).
He has the athleticism, ability, attitude, versitility, desire, and football IQ to step up to become a valued contributor in many different ways on offense and special teams for years to come. Losing Delanie was tougher on our offense than many care to admit, as a versitile athlete can tilt the scales to our favor in the run or pass game, depending on how the defense chooses to account for him. Trey is the man to step up and take on this role when healthy, and I absolutely cannot wait