We need a midfielder to free up Pogba (Fabinho/Dier), and a winger to compete with Martial/Rashford/Mkhitaryan. Perisic makes sense. Play him as first choice left wing, and it allows for Rashford's development to continue as it should (without overarching pressure to be the main man at 20).
Obligatory muppet fantasy XI:
--------------De Gea
Valencia--Bailly--Lindelof--Shaw
----Fabinho--Herrera--Pogba
Mkhitaryan--Lukaku--Perisic
Romero, Darmian, Smalling, Jones, Rojo, TFM, Tuanzebe, Fellaini, Carrick, Lingard, Mata, Rashford, Martial, etc as backup, depending on who we keep/sell.
Rashford and Martial occupy the Ronaldo/Rooney slots of our '04/05 squad, though neither is likely to feature as much, or have an impact anywhere near comparable. But each can play across all three forward positions, with Mata competing for the wide roles, or playing as a 10 if we go 4231. We're not close to that level, but we're getting there, provided we continue on the course we've set out on since Jose's arrival.
We've removed Rooney, Zlatan, and, in all likelihood, one of Jones/Smalling and Januzaj from the wage bill (as well as Schweinsteiger and Schneiderlin last season). I doubt Perisic, Lukaku, Fabinho/Dier and Lindelof combined would earn half of what we've shipped out over the last year (pure speculation of course).
Anyway, assuming we do persevere with Shaw, and buy Fabinho/Perisic, the average age of that above muppet fantasy XI would be 25 (or thereabouts), making it the youngest starting XI in the league (we also, on average, had the youngest starting XI in the league last season, yet people relentlessly complain about signing a 28 year old winger as if it would somehow render the rest of our squad geriatric).
That's a title winning team if Shaw can play to his potential, otherwise we either persevere with Darmian as the defensive LB, or invest in a new LB.