留我做當頭 Left Behind As A Pledge
三藏大驚道：「徒弟啊，此意是留我做當頭哩。若醫得好，歡喜起送；若醫不好，我命休矣。你須仔細上心，精虔制度也。」行者笑道：「師父放心在此受用，老孫自有醫國之手。」三藏大惊道：「徒弟啊，此意是留我做当头哩。若医得好，欢喜起送；若医不好，我命休矣。你须仔细上心，精虔制度也。」行者笑道：「师父放心在此受用，老孙自有医国之手。」
逐句解 Line by line
Tripitaka was badly startled. "Disciple," he said, "what this means is that they are keeping me here as a pledge.
Tripitaka reads the arrangement correctly and instantly, and he names it in the language of the pawnshop. 當 is dàng in 當頭, a pledge or security — the sense 康熙 files under its falling-tone reading, glossed there as putting an object down against money. Not the dāng of 當值, being on duty, a moment ago.
"If the cure succeeds, they will send us off rejoicing; if it fails, my life is finished.
His fear runs from other people's rejoicing straight to his own death with nothing in between, which is Tripitaka all over. 得 is the light de that links a verb to its outcome — 醫得好, to treat successfully — not the full verb dé, to get.
"You must be careful and give your whole mind to it, and prepare the thing with reverence."
仔 is zǐ in 仔細, careful, not the zǎi of a young animal. 制度 is not the modern "system" but the making and ordering of the medicine itself — prepare it properly.
Pilgrim laughed and said, "Set your mind at rest and enjoy yourself here, Master. Old Sun has the hand that cures a kingdom."
醫國之手 — the hand that cures a kingdom — is an old phrase for the statesman who heals a state. Pilgrim means it literally, about a patient, and lets the grander sense stand there unclaimed.
Tripitaka reads it correctly and at once. He is not being honoured, he is being held against delivery, and the word he reaches for comes straight out of a pawnshop.
His fear is exact, reasonable, and entirely characteristic — the two halves of his sentence run from everybody else's rejoicing to his own death with nothing in between. Pilgrim's reply does not address the risk at all. He changes the subject to his own competence and laughs, which is the form reassurance always takes between these two.