As per p. 121, treat all components in the leg as non-functional, but not destroyed. Therefore the leg is not considered destroyed, but a previously undamaged 'Mech would suffer the cumulative effects of a hip, upper leg, lower leg and foot criticals - with associated PSRs to remain standing. In following turns, apply only the effect of a hip critical (as per TW, p. 127) - the leg is considered to be locked in the straight position.