According to this article, athletes who don't get enough protein not only experience slower recovery and slower improvement, they're putting themselves at risk for injury and illness (the latter because the body needs protein to manufacture white blood cells).
...it is the position of the International Society of Sport Nutrition that exercising individuals ingest protein ranging from 1.4 to 2.0 g/kg/day. Individuals engaging in endurance exercise should ingest levels at the lower end of this range, individuals engaging in intermittent activities should ingest levels in the middle of this range, and those engaging in strength/power exercise should ingest levels at the upper end of this range.
Speed comes from what you put behind you. - Judi Ketteler