Trippier is a guy who played ~9 league games in 2 years before Walker told them he was leaving, and every single one was because Walker was unavailable.
He is a player Pochettino has only ever used in league cup or Europa league games when he's chucking out the second string and doesn't care about the result.
The idea that Trippier is an able replacement who will step up and fill in for Walker, without much of a drop off (he's even a better crosser!!) is complete and utter bullshit, it's come about because of a couple of good performances against shite opposition at the tail end of last season, but he got obviously exposed against any decent winger.
If you want to see the difference just watch the most recent United game, where Anthony Martial looks world class against Trippier and bang average/inivisible against Walker.
Pochettino knows it, at least, which is why he's looking to spend 30m on Perriera if Walker leaves.