For the sake of making sure this discussion doesn't leave anything out, the Lions, to the best of my knowledge, have 11-12 WR on the Roster right now that are worthy of having their names listed in the MFL player database for drafting, or here on the FBG's depth chart. Included are several guys that are likely duds.
The 2 most notable veteran names, Tyrell Williams and Breshad Perriman are likely going to be the starting outside WR's unless they get hurt, or the Lions add another noteworthy outside guy.
St. Brown is a slot guy, through and through, so it's not like he's directly competing with them for PT, which works in his favor. He will, however, be competing with Quintez Cephus, also a Day 3 pick (2020, ARStB went 4th Rd, 2021). Cephus had a monster 2019 at Wisconsin, coming off a missed 2018 due to sexual misconduct allegations he was battling at the time. I read plenty that even though those allegations were over a year old, they were still going to impact his draft position, potentially knocking him out of Day 2. IIRC the highest projected Round I saw him going in, in 2020, was Round 3 (Day 2). I read some stuff which regarded the Lions getting Cephus in the 5th as a value pick by 2020 season's end.
He will also be competing with Geronimo Allison, who signed with the Lions in 2020, pre-draft, on a 1/1M prove-it deal, and then took the C19 opt-out, because his wife had just had a baby when the whole C19 mess was becoming a thing. Allison can play both inside and outside, but I've read analysts who feel like his best position is big slot, and that the Lions signed him to play that role with Golladay/Jones on the outside for 2020...so he was their initial plan there, and they got a great look at him, while he played for the Packers from 2016-2019 in the NFCN. Allison's contract doesn't really effect the Lions, cap wise, so he's both easy to cut or keep, and he has no draft capital whatsoever, being a post-draft UFA signing by the Packers, but he's an NFL player who overcame tremendous odds as a UDFA to make a roster and be a key contributor for a Team known for selecting and developing WR talent. He's also a very good run blocker, both willing and effective, and the Lions certainly look to be built around the run.
I'm sure Cephus and Allison aren't scaring many off of drafting St. Brown, and that's not necessarily what I'm bringing here. I am saying he's not necessarily a lock to dominate the slot for the Lions in 2021, as his competition is roughly the same level of player as he is...but he certainly might be. So might Cephus or Allison. In 2021, there might not be much of a difference between those 3 guys, and if they're all on the Team, they might eat each other up. I do think he's a worthy dynasty stash, for the same reason (Cephus and Allison aren't scaring anyone), and he could be the primary slot guy at some point this year or in 2022.
What I am saying, is, if you like the guy's long term outlook, and don't wind up being able to get him in your rookie draft, a buying opportunity may present itself. If Cephus and Allison give the fight I think they might be able to give him this summer, he might be able to be acquired from someone who is discouraged enough initially to move on.
I don't have anything against the kid, I just think he's a typical Day 3 guy, competing with guys similar to him, and I don't think the 2021 role is as wide open as many folks in here apparently think it is. YMMV.
