That’s a pretty confusing post mate.
You say Sarr is good at winning the ball back pressing and chasing down after a pass in transition but not good at preventing transitions. If you win the ball back in the press that’s the second best way to stop a transition. If you are great at chasing them down after a pass that’s the 3rd best way.
The best way is not to give it away in the first place but that’s on the forwards more than it’s on Sarr and Gallagher sure as fuck doesn’t stop our forwards giving it away as often. He’s probably as good as Sarr winning it back in the press or even slightly better but several levels worse at chasing it down after a pass.
The first principle to be established/agreed upon is do we have a problem with Oppo Transitions? I think we all can agree that we have. For me this is the BIGGEST ISSUE we have as a team right now. (Set-Pieces are probably 2nd on the list but just hearing the PGMOL deciding to actually apply a law written in the rules will probably have gone some way to addressing this problem Yay!) But reduce the number of oppo goals scored from their transitions and we become a top 3 challenger.
To be clear - I like Sarr, the energy he brings to the team is what we've lacked in midfield for a long time, he provides 3rd man runs, his high-pressing is good, he is young and miles from a well-rounded player though, lots of development to come I'm not overly convinced with his technical ability but I believe he was very important for us last year. Loads of plus points to his game.
However, does he address our biggest weakness? NO, he absolutely does not - he plays in the team, in the actual key position employed to nullify oppo transitions and we are so, so bad at stopping them our biggest weakness. Again, I want to avoid scapegoating him regarding this, primarily this is a system thing, the way we have chosen to play will allow for spaces in midfield if the oppo successfully transitions against us, so there has to be an acceptance of this, it will happen (just as if you analyse the goals City concede they are predominantly counter-attacks) so it's about addressing the problem to the extent that we don't lose games because of it (which we absolutely are). And to be clear, he's NOT great at chasing them down when they occur, like all defensive players when the transition is more than two passes old it's probably too late, the oppo would have progressed the ball already 30yrd up the pitch taking our entire team other than Biss and our 2 CB's out of the game, Sarr (and others are in reality chasing shadows in this scenario he's probably stretched out a last ditched leg and stopped 2 all season this way - unless someone can post actual evidence that it's more I'm sticking with this made up number, but I'm genuinely struggling to recall any more than a couple successfully thwarted all season).
So, how do we stop oppo transitions? The ONLY way is to prevent them from occurring (again impossible to stop all of them the way we've chosen to play). But I disagree with you that this is on the forwards in that they shouldn't lose the ball as much, I think this unrealistic as ALL forwards lose possession of the ball more because they are always outnumbered and are trying higher-risk passes, flicks , crosses etc to get past oppo defences (having said that the arrival of Solanke, who's great at holding the ball up, great at bringing others into play and is a superb presser and allows for Son to drop into his best position will 100% make our ball retention higher up the pitch better - we already possess one of the best high-pressing forward lines to which Sarr is a key element).
The realistic focus to prevent oppo transitions is to recognise when one is on for the oppo, this involves jumping on the oppo player that's going to play the first pass AND/OR the receiver of the first pass. Sarr's position demands that he should be the man doing the higher % of this, to add some balance I've not seen any evidence from any of our players other than perhaps Udogie & Bentancur before his injury (Lo Celso was actually very good at it when he's actually on the pitch) who's jumping on the key oppo player in the first phase of their transition. And when I say jumping I really mean jumping, fouling, smothering, slapping, elbowing whatever it takes to disrupt this phase of the oppo transition. Purely as a point of reference and by way of an example this is where Gallagher excels and why I imagine Ange is supposed to be a fan (as was Poch). We can instantly reduce the biggest problem the team has (by enough margin that turns draws into wins and losses into draws - I can think of 4 or five losses last season where they would have been wins - an improvement of 15 fucking points!!!) by having one player who does everything that Sarr currently does but does one thing that Sarr doesn't. Again, it's not specifically saying Sarr is shit but he plays in the position that should be contributing the biggest input that deals with our biggest weakness - our biggest weakness exists with him playing).