Traditional wisdom - and certainly my inexpert instinct - says that a player whose main qualities are ball retention and progression is probably going to be more effective centrally, because it opens up more angles for runs and passes than are available on the wing.
He's never going to be able to consistently go round defences on the outside because he doesn't have the sharp burst of pace or tricky left foot that would require (he has pace but it takes a few yards to build up, and his left foot is functional but nowhere near the skill of his right). He can cut in on his right, but when that's his only option and there's no overlapping runner on the left, it's pretty easy to defend against. Whereas if he was central, he'd have passing options on either side of him as well as spaces (even if they were tight spaces, which he's pretty good with) to run into.
So by that logic I'd love to see him lined up as cover/eventual replacement for Gundo (I love Gundo and he'd be my preference in that left 8 position over Jack right now, but his age and injury history mean he can't play every match and his contract situation means he's unlikely to stick around for many more seasons, so now is the perfect time to develop his replacement. Bernardo would still get plenty of minutes rotating the right 8 and the right wing).
But Pep isn't following that traditional approach right now.
Instead, it seems that what he's doing is asking Jack to hold onto the ball on the left, drawing defenders to him, then switch the ball central or to the right wing, so his teammates can exploit the space those defenders have vacated.
It's a task that requires patience and perseverence and, unfortunately for Jack, it's a largely thankless task because even if he delivers his part of the plan to perfection, success is dependent on what other people do with the ball after he's moved it to them, and of course the credit for a goal goes to the scorer and assister, not the guy who made the space for it to happen. He can get into goalscoring positions by moving into the box after he's moved the ball out right, and of course he does need to do better in those situations.
If we're playing this way, I prefer Jack on the left and Phil on the right, because Phil exploits that space well with speed and aggression.