I feel like Stubbs has already acquitted himself well. I get not making too much of intangibles or the fact that he's as much a bullpen coach as a player but that might at least be worth the 0.2 WAR a better back-up catcher gives you. He's 0.3 WAR on BBRef, -0.1 on Fangraphs.
Marchan may also actually be better than most of the back-ups currently in MLB. Just not proven.
What I meant by more certainty not less, is a surgically repaired JT is probably a better bet going into September than one with a chronically sore knee. And if they are forced to play him less that's also good. Paradoxically, he's also a lot more easily replaced than he would have been two years ago, both offensively and defensively.
He must have reaggravated it or newly tore it in London, maybe even at that moment. Obviously he'd played through it the first time.