#9 is fair. I don't hate it, actually -- and I was afraid the UFC was going to make him #1 contender or something. I could make an argument for as high as #6... but probably not any higher than that. I guess you could also go lower if you were so inclined.
So in general when a ranked fighter transitions from one weight class to another, I view it sort of like an exchange rate -- like bringing your cash to a new country where it means ever so slightly less. Or, for any veterans out there, if someone transfers from one branch of the military to another -- they sometimes lose grade/rank in the process. Similar deal with ranked fighters -- ideally when they transfer to a new division, they should be fighting a dude who is a few slots below the number they held back in their native division. So on-paper, Chimaev -- a Top 3-5 guy at Welterweight for quite some time -- fighting Costa who was just outside of the Top 5 up at Middleweight made a lot of sense for his first test.
But Costa pulled out and the UFC threw another ranked Welter into the mix, so things got messy. Chimaev deserves to be rewarded for beating a legit dude at the agreed-upon weight and shouldn't really be punished for Paulo not showing up... but at the same time he didn't beat a ranked Middleweight contender. So what you're left doing is trying is sort of average the perceived "value" of the short-notice Usman (repeat the process above here) win alongside Khamzat's old Middleweight wins against GM3 (longtime fringe Top 20ishguy) and John Phillips (meh). If you're generous you can also include Holland, since it happened closer to Middleweight than Welterweight and Holland was a Top 10 guy at MW a couple of years ago. You could also try to factor in whatever you think the value of his win over Ikram outside the UFC is.
Is the end result of all this a #9 ranking? I have no fucking clue... but it's not terrible, all things considered -- and I'm one of the few idiots who actually is kind of a stickler about rankings and cares about those sorts of things.