Consider the irritability of the patient's symptoms in acute inflammatory phase. Irritability in a nutshell describes how easy it is to provoke the patient's symptom and how quickly it abates after the aggravating factor is removed. e.g. a highly irritable condition is easily provoked and takes relatively longer period of time to settle the symptom. High irritability is more common during the inflammatory phase and therefore manual therapy is usually avoided, in another word, you don't want to make the patient worse.
Of course, not all inflammations are of the same magnetitude and therefore irritability. In the scenario of hamstring strain, you would probably carry out manual therapy if the patient has low irritability, even during acute phase e.g. in a low grade strain. Whereas if the patient can not bear you touching the injured area, you would reconsider manual therapy.
I hope this makes sense as it is not always cut and dry in clinical setting. It would be useful to take irritability into account to make these clinical decision vs. following a set protocal in managing injuries.