Maybe the decision isn’t that hard after all
Despite Happ’s strong start, it is unrealistic to think he will stick on the big league roster. Both Jay (.296/.405/.366 in 33 games) and Albert Almora (.271/.326/.400 in 35 games) are off to strong starts while Heyward is showing signs of a bounce back year at the plate.
The only outfielder not performing well so far this season is Kyle Schwarber. The 24-year-old is slashing .188/.312/.361 with six home runs and 17 RBI in 37 games. Perhaps, if Schwarber continues to struggle, he and Happ could platoon left field. That would be a tricky action as Schwarber is the lead-off man for the Cubs, and finding a good choice to replace Schwarber atop the line-up three days a week would be hard.
Taking all of this into account one can see that sending Happ back to Triple-A once Heyward returns would be the best option. Happ, as well as Almora and Schwarber, need regular at-bats if they hope to progress into better hitters.