It's fun to speculate on the different scenarios, but the reality is they all change as injuries happen, etc.
Urshela hasn't actually played 1B this year. Only three games in the majors, two in 2017 and one in 2019.
But the reality is that with Miranda's struggles, 1B is the position that has by far the least flexibility right now in terms of demonstrated ML games. Best I can tell, Arraez is the team leader (11) in career games played at 1B in the majors, followed by Miranda's 8, with a .348 OPS. No one has played significant games in the minors either, unless you count Miranda.
So, my .02 worth. They play today and tomorrow, followed by an off day and then a stretch of 18 games in 17 days. That will require particular flexibility on the pitching side, so they may be hard-pressed to keep just 13 pitchers. Right now, there are 12 hitters active.
Rocco was quoted as saying Correa won't be back today, but "I think we could see him out there this series." Gee, that leaves Wednesday. Miranda is the obvious send-down in performance, but that leaves only Arraez at 1B, with others as the emergency backup. If you don't like that, you have to keep Miranda, so your options are to send down Lewis or Garlick. Doing the latter means that you only have Buxton, Kepler, Gordon and Celestino in the OF, and you're committed to giving Buxton days off. Or you could send down a pitcher and play a few days with 13 pitchers.
But in the upcoming stretch, even in full health, my hunch is that Correa would have gotten at least two days off in the 18 games in 17 day stretch. Coming back from injury, perhaps more. If activated, he'll play Thursday, but perhaps two out of three over the weekend, in which case you want Lewis around. Oh, and how close is Larnach?
In my mind, a lot of it comes down to 1B and the comfort level of having only Arraez and an emergency back-up 1B there. I personally take my chances and send Miranda down. In the four days after today, I plan on four days of Arraez at 1B, three days of Correa and one of Lewis at SS. Lewis gets that day at SS, two as DH and a day off. By that time, another IL move will happen -- it just does. If so, Miranda's likely back up, even with the hitting struggles.
In the meantime, Urshela's getting ground balls at 1B. I don't see them using Lewis there. Long-term, there's no chance he spends significant time at 1B, so if you're going to get him in the lineup at another defensive spot, make it be one he could conceivably play with regularity if Correa stays around another year. To me, that's only 3B. We mention LF, but we've got other options there and it's next on the defensive scale of easyness, so you don't waste his talent there. Lewis's only real experience is at 3b was in Arizona, but by all accounts, that went well. If he's even close to Urshela defensively and you have to play someone out of position as the backup at 1B, make it be the rental. Oh, and hope Miranda or Kirilloff regroups enough to be able to play 1B.
But assuming no one gets hurt today, we'll know soon and then can resume bashing Rocco on how he handles the pitching staff. (Sarcasm alert on the prior phrase.)