That's the straightforward approach, but if you think Merrifield can hit RHP and he's the first guy off the bench to PH and PR (as well as someone you don't have to sub for once you do that) you could get by without a LHB for 4-6 weeks if you don't want to lose Pache until you're sure Rojas sticks. You're probably only talking about 3 or 4 PH ABs for Cave (or whomever) in that time frame.
The question is, do you want to see a Jake Cave/Brandon Marsh OF on Opening Day against Spencer Strider, or would you rather see Marsh/Rojas? I'm assuming the latter. If you're going with Rojas and you're assuming he helps the team win even on days he goes 0-5 (especially since Jake Cave is also capable of going 0-5 and you lose defense at both spots) may as well let him sink or swim against all RHP too. I guess that is also how Schwarber ends up playing LF five or eight times.
And then Marsh sits for Merrifield against a lefty once or twice a week.
Thomson has said they think Pache can hit RHP. As with Merrifield, it's a premise based on optimism - that Pache is a different player than he was before joining the Phillies, who can finally put it together. But if the team is healthy Pache would just rarely play. Much like Castro late last season it would be a quasi-Rule 5 situation. If there's injuries, different situation, but then you could also call someone up (possibly Jake Cave himself). It would just be hard to find actual starts for Pache.
I'm still wondering if Marsh will really be ready on Opening Day. (Or, if he's in the line-up, if his performance will show he isn't.)