There's an argument for both of your perspectives I think. In the short term, yeah, it is just one game, hopefully his struggles earlier this season were also due to the first injury, and he's a proven postseason pitcher which we need more than ever with Wheeler down.
But at the same time, it's true he has been trending down somewhat - stuff, adjusting to the pitcher timer, pitching with men on base, home runs. And now this season he's lost his single biggest atttribute - innings and durability. Doesn't mean he won't get back that too, but who knows.
I wouldn't say it was worse than the Howard deal because Howard got his deal two years early and was already cooked when the extension started. Nola was on the open market, and he did already give us a very good season last year (3.7 WAR). And if it turns out he's only an overpaid 4th or 5th starter for us for the next 3 years, that doesn't really bother me. He gave us a bargain on the last contract. Same if he's unpitchable in Year 6 and 7 because that's the price of doing business. What we can't have is him being unpitchable from '26-'28.