Here's what I know for certainty - you want a piece of this backfield. If only one of these guys, Alfred Morris or Matt Breida is available in your league - that makes it easy, go get that guy. Here's the obvious reason why:
He is just a goddam wizard when it comes to churning out good fantasy RBs. It's just the way his system is set up. There's no arguing it, and that's why it was nearly impossible not to be high on McKinnon prior the calf strain and now this ACL tear of course.
But what happens if both guys are available in your league - or your draft is today, or tomorrow whatever. Let's break down both guys a little bit.
Alfred Morris
- Last year, Morris ran for 547 yards on 115 carries (4.8 YPC). Kyle Shanahan knows him, trusts him.
- #10 DVOA per FO
- 3.3 YAC ranked 5th in the NFL among 73 RBs with 50+ carries.
- He played under Shanahan back in Washington, had multiple 1,000-yard seasons with him and knows the system well. He is 29, but age usually becomes an issue due to the number of hits backs have taken, you see a guy like shady who might scare you off because of his age, he has like 9000 carries, Morris has 1,262 - no spring chicken, but not as seasoned as these other 29-30 year olds. Plus he only has 184 over the last two years.
- The beat reporters out in SF have Morris ticketed as the starter, which doesn't surprise me. But that literally doesn't matter guys, don't get caught up in the "starter" talk - whichever play Shanahan has chalked up to be the first play of the game will dictate who the starter is.
Matt Breida
- An UDFA out of Georgia Southern kind of burst onto the scene last year. As a rookie: 105-465-2 (4.4). 36-21-180-1
- Missed preszn with shoulder
- #5 DOVA per FO - just an overall good rookie season that many people didn't expect coming.
- The question becomes, how much more does Breida's role actually change with McKinnon gone? It seems like the popular opinion is that Breida will play the Tevin Coleman type role, while Morris operates as the lead back. Which makes sense, given his size 
He's 5'9 - 195, he's not going to handle a full workload, but Morris who is much bigger, and has done so in the past, can.
It's really close for me, and I'd try to get at least one of these guys in any league I'm in. But I look at it this way. Why were we so excited about Jerick McKinnon to begin with? And what part of that translates to the new RBBC. The reason I loved McKinnon wasn't that I thought he was getting 250-275 carries. It was going to be his involvement in the passing game. Last year, Shanahan turned Hyde into a full-on stud in fantasy, he caught 59 passes on 88 targets. Hyde caught 59 passes last year. Morris has 57 career receptions, in 6 seasons. Alf played 1362 snaps with Kyle Shanahan in Washington. Totaled 26 targets. (Per Scott Barrett of PFF) He was the Jordan Howard, before Howard existed. The reason I love McKinnon was his involvement in the passing game, because Shanahan understands that throwing to your RBs, especially on early downs, is a huge way to gain an advantage on teams. Looking back to last year, SF under Shanahan targeted their running backs on 28% of their passes - that was the 2nd highest rate in the entire NFL - behind only the Saints. I expected that to be the case in 2018 as well, which is why I was so high on McKinnon - could care less that he would get 190-220 carries.
Morris has 20 receptions over the last three seasons, Breida had 21 just last year in a limited role. However, I'm not sure Breida is actually an amazing pass-catcher and they don't just try to give Morris some of that work, maybe 35-40% of the RB targets, which would be huge for fanatsy purposes.
A huge part of this offense is going to be throwing to the RBs, so for that reason, I think the obvious choice in PPR leagues is Breida, for standard it's Morris, for 0.5 PPR I'm a bit torn tbh - I put in a $21 FAAB bid last night and got Morris in one of my leagues, put in a $20 for Breida so I guess realistically it says that I'll choose Morris in 0.5 PPR. So at the end of the day give me Breida in full PPR, give me Morris elsewhere. Get one of them either way if you an - I wouldn't blow the bank on either one of them - I think 20-25% of my budget is the most I would spend.