I honestly think that's pretty much impossible. It really is slim pickings already, including from the already narrowed down candidates below...
With the sad news of Casemiro leaving , combined with the sad loss of fbref, I thought I'd try and use some of the other tools I've since discovered to try and find possible replacements. The most interesting tool I found, which I've used for a lot of the following analysis, was this rather temperamental one:
https://best11scouting.streamlit.app/Men's_App
I also cross-referenced those findings with the stats and visualisations on other sites, eg
https://datamb.football/midfielders/ and
https://www.fotmob.com/en-GB/players/208494/casemiro.
I've focused on the key attributes for a #6 /DM that Casemiro brings to the table, and which should be the minimum requirements for United's needs, namely:
strong ball winning ability, and
strong forwards passing/play-making from deep. Were I to have also tried to factor in some of Casemiro's other strengths, such as aerial ability and goalscoring, then it would have removed far too many possible candidates already. The field is narrow enough as it is even just focusing on DMs who are both solid at ball-winning and forwards passing. Considering United might have someone like Mainoo as an 8 who lacks pace and ball-winning ability, I really think that strong ball-winning stats can not be compromised, e.g. someone like Wharton would be too defensively weak as a 6 alongside a Mainoo.
The final combination of parameters I settled upon resulted in the following list of matching players from the Premier League, which I think is a good demonstration that it is broadly focusing on the correct profile of player, even if some of them may have a couple of things which mean they are not ultimately United quality, or suitable for United's needs.
I have bolded those who seem to stand out as potentially possible and sensible. I left all the PL players below for demonstration purposes, but from other leagues I removed most of the older (27/8+) and notably lesser players to save space as they are unlikely to ever be under consideration.
(TLDR, only ones even close to consideration worthy, but all would be initial downgrades: Yarmoliuk, Anderson, Ampadu** Janelt, Chema Andres, Seiwald, Casado, Barrios, Ederson, M Sangare)
Premier League results (age order):
Yarmoliuk - Brentford - 21
Anderson - Forest - 23
Nico Gonzalez - Man City - 24
Caicedo - Chelsea - 24
Ampadu** - Leeds - 25
Janelt - Brentford - 27
Ibrahim Sangare - Forest - 28
Bruno Fernandes - Man United - 31
Casemiro - Man United - 33
Xhaka - Sunderland - 33
Anderson is the most obvious potential replacement out there, with some of the best overall CM stats in the PL. No doubt he'll plenty of suitors in the summer. He can as a 6 as well as an 8.
Yarmoliuk stands out as being another very promising young candidate not already at a big club. I don't think he'll be ready yet to be first-choice at a club like United, but given what he's doing so far at 21, and the improvements he's showing, he's not one to rule out in future. He could also be a good second-choice 6/Ugarte replacement. He doesn't have Casemiro's aerial ability unfortunately.
Janelt is having a very strong season so far for Brentford, and is very good in the air, but his age and potential fee would probably rule him out.
Ampadu** is a curious one. I've bolded him as I've just noticed he'll only have 1 year left on his contract in the summer, so could be a useful inexpensive squad option/Ugarte replacement. Not as obviously elite all-round as Casemiro, or Anderson, but as a pure ball-winner in the 6 position his stats are right up there for the Premier League. He certainly has plenty of pace and mobility, and is a good age. His passing is also surprisingly solid, even if he lacks the creativity and flashiness of the likes of Anderson and Wharton. He might get less hype as he plays for Wales, rather than being in the England conversation.
Quote from Leeds' forum: "If Ampadu and Aaronson, in particular, maintain the level of play they've attained in the last couple of months they will attract a lot of attention".
I think it's worthwhile adding a note that Garner scores very well on some of these measures, but where he gets cut-off by my criteria is "Accurate Long Pass %" - his figure for this looks really bad. I don't know whether set-pieces are a factor in this (but they don't impact Anderson's and Bruno's numbers in the same way if so) - but the data has him close to Ugarte for giving the ball away when trying longer passes. Seems he loses the ball too much with too many Hollywood passes that go astray. Short-medium passes he seems fine.
Bundesliga:
Chema - Stuttgart - 21
Kemlein - Union Berlin - 21
Avdullahu - Hoffenheim - 21
Seiwald - RB Leipzig - 24
Chema is the clear stand-out here from his stats. He is also strong in the air, which is rare for his age and position. Real Madrid will surely use their buy-back clause though. The others have too many deficiencies to move at the moment, but could still improve.
Seiwald seems a solid, if unspectacular, defensive-minded player.
La Liga:
Marc Casado* - Barcelona - 21
Pablo Barrios - Atletico Madrid - 22
It's funny to see Man United linked recently with
Marc Casado, as his tackling and passing stats look pleasingly close to Casemiro's. He is on the small side though and lacks aerial ability, but at least has a bit of pace and goal threat. His stats also only tally for his 24/25 season - he hasn't really played enough this season, with Eric Garcia being favoured right now.
Pablo Barrios isn't as defensively solid as some of the others, but has a bit more mobility and creativity than many. Would probably be too expensive though with a recently extended contract at Atletico.
Serie A
Ederson - Atalanta - 26
Locatelli - Juventus - 28
United linked with Ederson a few times before. Seems very much a jack of all trades, master of none. Seems a solid all-rounder, without being better than Casemiro at anything.
I've added Locatelli back to the long-list as his stats are very good and he has a good profile, but I reckon his age would contribute to him being a non-starter.
Ligue 1
El Mourabati - Strasbourg - 20
Avom - Lorient - 21
M Sangare - Lens - 23
Tessmann - Lyon - 24
Mamadou Sangare and Avom have been mentioned before on here, and United have been linked with Sangare recently. Sangare's ball-winning stats look elite (in Ligue 1 at least), and his passing doesn't look too shabby either. It's only his first consistent season at a decent level though - United could be a big jump for him.
As an aside, El Mourabati looks an another annoyingly promising player on the Strasbourg/Chelsea production line.
Jupiler Pro League
Van De Perre - Union St Gilloise - 21
Seems a strong ball winner who I wouldn't be surprised see move to USG's sister club Brighton.
Eredivisie
Regeer - Ajax - 22
Liga NOS
Juanse Rodriguez - Fiondela - 19
Barrenechea - Benfica - 24
Juanse Rodriguez is an Ecuador u20 international, only recently moved to Portugal from LDU Quito. Expect a move to another stepping-stone club soon.
Not sure why it's not worked out at Villa for Barrenechea?