It all revolves around Baez
Let’s assume Baez will start at second base. That takes away the possibility of Zobrist and Happ getting the start at second. It’s safe to assume La Stella will be a pinch-hitter barring an injury.
Coming into the season, it was expected that Almora and Jay would share time in center field. But Happ has emerged as a solid bat in the middle of the lineup and played well enough defensively to not become a liability.
Almora has proven to hit very well against lefties (36-for-107 .336 batting average). As of late, he’s been outstanding against right-handed batters, but overall, he’s struggled against righties for the season (48-for-178, .270 batting average). His defense is what puts him above Jay or Happ.
Better than expected
Jay has been better than anticipated for the entire season. He’s been a great pinch-hitter while also coming up with a lot of big hits for the Cubs. He’s arguably one of the Cubs most valuable players in the first half of the season as mainly the Cubs lead-off hitter. He slashed .300/.385/.400 in the first half of the season. In the second half, he’s slashed .278/.366/.344. There’s not much of a difference there. He’s earned the right to start.