Bailey needs to see a ton of duty in the slot in the absence of Tavon.
I am also mystified as to why we ran the end around with Cook, and not Bailey if Bailey was playing.
I think you can figure it out. You're a smart guy. I mean that sincerely.
People get tunnel vision when they think about these things. Think about the game in terms of personnel packages and match-ups. Cook is extremely fast for a TE but teams still have to treat him as a TE.
On that specific play, the Rams went heavy with 22 personnel(HB, FB, 2 TEs). Dallas responded by putting 8 in the box including 6 on the LOS. I'll post the pre-snap picture for you:
The Rams align both TEs and the FB to the strong-side of the formation which means Dallas must counter by aligning two LBs and their CB to the strong-side. That means there's only two players assigned to the weak-side of the defense beyond the DL(FS and CB). Brian Quick can block the CB. That leaves one guy to account for.
Here's why the play didn't work. I'll give the FS props in that he did a great job of maintaining his gap discipline on the play. However, he would have been no issue for us if not for Bruce Carter. Jake Long released too far up-field on the play. He was supposed to seal Carter inside and had the opportunity to do so. Issue was in getting too far up-field, Carter cut underneath Long and was able to set the edge and engage Cory Harkey. This strung the play out and didn't allow Cook to cut up-field until he was nearly run out of bounds. It was a great play by Carter and a mistake by Long. If Long doesn't get so far up-field, he seals Carter in and Harkey is able to lead Cook to the edge and block the FS. Which means that Cook has a lot of green in front of him.
You can see it here. Long is too far up-field which allows Carter to run under him and engage Harkey as he comes around the edge. He then continues to move down the LOS until he forces Cook outside the numbers. Had Long sealed Carter on the play, Harkey gets to turn that corner and the safety is going to have to avoid or defeat the block of Harkey. The unblocked guy next to Wells is a DT and no threat to Cook due to his lack of speed. This means that Cook can cut it up-field almost immediately and if Harkey engages the safety, Cook has a lot of open room. Again, play design is sound here. Just one missed assignment ended up ruining the play.