I won't say Helu without a doubt like some previous poster did, but it is absolutely his to lose and Castille will have to do some miraculous things to pass him on the depth chart.
I don't know how much you guys remember of Helu running, but when I watched him you could just TELL that he deserved to be on that field more than Lucky or Castille. I don't know what it is, but something about him running the ball just made you think "dang...this guy is seriously freaking good".
Also, take a look at the stats. Ever since he got the largest amount of reps (Oklahoma), he finished with 157 yards and a 9.8 average. Against Kansas he finished with 115 and a 7.2 average. Against K State he finished with 72 and a 5.5 average (and it could be argued that he would have had a much more productive day, if not for us destroying them and Ganz breaking free on a lot of option runs), and lastly against 166 yards with a 6.6 average. Those numbers are extremely impressive, no matter which way someone tries to spin them.