I think Tuchel, who knew Pulisic at Dortmond, thinks of him as more of a super-sub than a 90-minute full time winger. Bringing his pace and direct play style can really punish an opponent late in a game or get a goal late when you need it. There's a balance Tuchel has to play between Pulisic's injury history (and most recently his hammy injury after seeming to light the world on fire after the covid restart last August), and playing some home grown stars with lots of potential like Mount and Odoi. You want to play the $73 million transfer player your bosses acquired, yet you are also expected to oversee continued development from young home-grown stars.
Remember that Mason Mount has been in Chelsea's academy since six years old and Callum Hudson-Odoi since 7 years old. ..kinda makes you wonder why they paid so much for Pulisic to begin with. I think they saw the next Edin Hazard in Christian, and thought they'd need a few years to bridge the gap from selling Hazard before Mount and Odoi really came on.
Pulisic has looked good in moments he's played since Tuchel took over, but he has not been quite as dangerous or had the goal scoring he was enjoying before the injury. Now, he's not been rostered a time or two and his only start was an FA cup game... certainly concerning for him. It's not like the other wingers are scoring a lot of goals either.