This is the only way I can see Ricciardo realistically getting another shot at Red Bull. If Perez can't race this weekend and Ricciardo does well, he might be back in it.
Checo would push the limits of what's humanly possible to stay in the car to rule this out, even if, as mentioned below, DR is not the first choice replacement.
Also, I believe part of the reason Danny isn't racing this year is his contact with McLaren, he would lose out on millions of dollars if he raced this year. The specifics aren't known, but supposedly even racing as a reserve driver will cost him.
I don't think it was confirmed, and it was a while ago I saw it. But (alledgedly) the reason he turned down the options with too little pay, is that they had to beat the amount that McLaren was paying him not to race.
To clarify, this kind of depends on the race. Daniel isa reserve driver for Red Bull (also Alpha Tauri), primarily at the races he’s attending (of which there are eight, I believe). Lawson is presumably the default at the other races, but if Ricciardo is available they could well choose him instead.
But yeah, as Lawson and Ricciardo are shared with Alpha Tauri, they could also, for example, ask Yuki to drive the Red Bull and put, say, Lawson in the AT.
They have more options than just Danny Ric though. Jake Dennis has tested for RBR before and is in good form atm, leading the FE championship. Cassidy is currently part of Red Bull and his current form is insane. They also have Lawson as a test driver. Yeah Danny makes the most sense, but if you give a new driver a chance, you can see how well they fair and can have them replace De Vries if they do well.