Possibly his age has slowed his recovery?
My guess would be Brentford set his injury back and he's playing with fear now.
I'm sure there's research on it somewhere, but it always seems to me that when players reach their 30s it's the stops and starts that cause the issues as much as being overplayed/tired. Not just injuries, but breaks in play in general.
Like you're average 22-27 year old top level footballer goes off on their jollies at the end of the season, and let's say their fitness drops to 70% (or if you enjoy a Firmino-style off-season, maybe 50%). You come back for pre-season, work your way back up, and then early in the season you hit 100%. You'll start to drop as you get tired as the season wears on, but then you'll go on your summer holidays again, and the cycle repeats.
When you hit your 30s it feels like everytime you lose a bit of fitness it's harder to get it back, and you maybe don't even get all the way back to 100%. Maybe you drop to 60% in the off-season, but can only get back up to 90% when the season starts. Then you pick up an injury, and build your way back but max out at 85%. Then you take a break, lose some of your fitness, and this time can only build back to 80% after pre-season training. It's just harder and harder to pick the body up and go again at this age, and I imagine Salah is feeling it after what must be the worst injury of his Liverpool career.
It felt like it played out like that a bit with Henderson and Fabinho anyway, in the way they almost seemed to lose their legs overnight - they both showed signs of slowing down/increasing injury-proneness (in Henderson's case at least), but they went away for their break and came back in summer 2022 and just basically never got back to grips with the pace of the game ever again, and that was it.
I don't think this version of Salah is representative of his current true level, but I do wonder what his max % is, so to speak, as I'm not convinced it's a case of "rest up and go again" when players are this age.