He has form on the hamstring front I know he missed games for Leicester with this too, and speaking from a podiatry perspective we see people over and over with recurrent hamstring strains [usually biceps femoris]. This is simply because of the extensibility of the hamstring [if this] is not reinstated after injury then it is inadequate, and given that the hamstrings oppose the quads and have a similar relationship with the glutes which are some of the strongest/largest muscles in the body its easy to understand how they could give way especially if they are underdeveloped in comparison.
In the case of glutes, many people actually have a large muscle group but inadequate strength, and thus the hamstrings are recruited causing unnatural strain to the area too. Either way its the terminal swing phase of gait, IE just before the foot makes contact where the stresses are highest as the hamstring absorbs the energy of a limb that has accelerated but is now trying to decelerate whist the muscle lengthens in order to accept the whole weight of the body which is magnified up 7 times by running and also the concurrent ground reaction force which is equal and opposite. Add to this that a footballer may kick a ball several times during a match and whilst running places a toll primarily on the biceps femoris (hamstring] muscle it also affects the semitendinosis [hamstring muscle], and kicking can damage the fibers of this muscle because the hip is flexed whilst the knee extends placing massive strain on the semitendinosis which is therefore at the maximal extension it can reach, being placed there with the huge force of powerful kicking.
As Cellino sacks the backroom staff one by one, you worry that the rehab may suffer and the wrong exercises [eg progressive excessivestretching] can actually seem to be helping but make the situation worse by encouraging the formation of thick scar tissue that is bound in the muscle making it weak as neogenesis of muscle tissue can either not take place, or penetrate the area leaving a weak spot that is susceptible to future damage/injury.
Interestingly giving crutches means that yes you take the weight off the injured leg which is desirable but holding the knee in a flexed position for any length of time so that the foot is off the ground puts the hamstring's damaged tissue under contractile load which is exactly what you need to avoid.
Thanks Chris. Good luck :-}