Mo Salah got injured in the last match vs Leicester. Seems it is nothing too serious but he will be out for at least 10-15 days. Good thing for Liverpool is that there is now an International break and he has some more time to recover. I can't remember the name of the Leicester player but it was a very dangerous tackle that infuriated everyone.
I think he's knock just before the International break is actually a blessing in disguise for Liverpool, if it's a minor one. He'd get to rest properly after several consecutive matches. And moreover Liverpool also have series of tough fixtures lined up for just after break, they can't afford to have their top performers fatigued.
player rotation must be done to give players time to play too often, if forced it will not be good because of the potential for injury. All professional clubs must do something like this. Liverpool also play in the UCL competition and this certainly makes their players feel tired.