I'm not sure it's going to be as simple as Almora vs LHP and Jay vs RHP for two reasons. The first is obvious that Almora is the long term fixture here. LHP consists about 25% of pitchers in the majors. Sitting him 3/4 the time just doesn't make sense considering if things go to plan he will be your 2018 every day CF. The second reason is Almora had reverse platoon splits last season. He hit for more power vs LHP but he was considerably better in avg and OBP vs RHP at .262/.279/.548 vs LHP and .286/.324/.400 vs RHP. The slg is almost assuredly a sample size issue as that would be a .286 ISO for a guy who's really never been above .140 ISO.
Jay is some what platoon neutral. On his career, he has hit .284/.352/.354 vs LHP and .288/.352/.394 vs RHP. So, you're talking more power vs RHP but he's largely the same guy vs lefties. I personally would expect them to generally start Almora vs RHP. I think Jay's role might be more dynamic. Starting him against LHP obviously makes sense. He also can probably lead off. So, if we're working under the assumption that Zobrist is the primary lead of guy, Jay starting on days he's out of the line up also makes sense.
My first reaction to the move was "$8 mil? That seems like a fair bit of money." After it sunk in a bit and I looked over the numbers I'm actually really fine with it. $8 mil is basically a 1 win expectation. Considering Szczur was able to put up 0.7 fWAR last year. And that actually brings up an interesting subject of the OF depth. So presumably your starters are Schwarber Almora and Heyward. Jay is now one of your two primary back ups. Typically speaking, if you run 13 position players you only carry 5 OFers. There's talk of adding a 26th active roster slot. However, maddon has liked to use 13 relievers and 3 C's. If that indeed is the route they go down, you're only talking about one back up at infield and outfield. With them also having to find playing time for Zobrist/Baez, it would seem unnecessary for them to carry a 5th OF.
Obviously Soler is being discussed as a trade piece. But the side effect would also be having to move Szczur because he's out of options.
Jon Jay didn't take a one year deal to be a backup. I assume the plan is to have Almora work on his hitting every day at Iowa and then start him next year in CF. The Cubs, probably rightly, looked at the advanced numbers on Almora and thought his swing wasn't ready to play every day and get 400 PA. Even with a Soler trade, I think the next man up in terms of playing time would be Candelario and then having Bryant play more in the OF (or having Jeimer learn the COF).
Almora is still young (he's 23 in April) that there is no real reason to rush his progress. He's still only at 450 PA at AAA+ so another year in AAA isn't the worst of things for his development. And with the talent and depth this team has, I just can't envision Almora ever having the opportunity to be a consistent lineup option for this team. The Jay signing also might mean the Soler market is so weak right now the Cubs don't feel there's any value to trading him. Sure they'd move him for value but they won't just move him to do it. And if Soler is here, then the opportunity for Almora PA shrinks. I mean, the Cubs have probably 12 guys right now who could probably be 3+ WAR players with 500 PA (Bryant, Rizzo, Schwarber, Russell, Baez, Zobrist, Heyward, Jay, Almora, Soler, Contreras, Candelario) so if you can keep guys down in Iowa to continue to develop, why wouldn't you?
Almora will be back here with an injury to either Heyward or Jay so it's not like he's been banished from the roster.