Obviously, protein is an essential component of any diet, and is in higher demand for the athlete, whether endurance, speed or power oriented. Proteins make up most of your tissues, like muscles, skin, or blood, and also are what make up the enzymes and some hormones needed for all types of metabolic processes, obviously including those associated with exercise.
An adequate and steady intake of protein is necessary to make sure that the raw materials are available whenever you need to build up new tissues (e.g. new muscle tissue, new blood, or new metabolic enzymes), whenever you need to repair damaged tissues (e.g. sore muscles or injured tendons), or to a certain extent, to limit the catabolism of your own proteins for metabolic needs (e.g. to limit the breakdown of protein for fuel during very long training sessions when other fuel sources are not sufficient).
Any balanced diet that includes a good mix of vegetables, fruits, legumes, lean meats, dairy, whole grains and nuts will provide enough protein to meet your needs as an athlete. Generally, any athlete with a balanced diet will consume enough food simply because of caloric/energy demands that he/she will get more than adequate protein on a daily basis to meet their needs.
It’s good to point out, though, that a focus on taking in a lot of extra protein from animal-based sources will probably be counterproductive to the athlete’s end goals. They are great foods to have in moderation, especially because they can be excellent sources of nutrients not readily available from other sources (most notably vitamin B-12 and readily absorbed heme-iron). But, animal-based sources of protein (e.g. fresh meat, fish, cheeses, processed meats) as well as many grains and refined sugars and starches (e.g. breads, grains, sugary breakfast cereals) are usually acidic foods that are totally fine to eat, but it would stand to reason that they will be best balanced with fruits and vegetables that will be good sources of minerals and will make it easier for your body to maintain its naturally alkaline pH.
Most of the stresses of training will lower your body’s pH (i.e. make it more acidic) and among other things, a lower pH will limit performance and increase protein breakdown in your body. Lower pH and muscle wasting is not good for athletes, obviously, and one might note, is something that happens to people as they get older. It is likely that having a more alkaline diet may be both good for performance and good health in general. Though some may debate the importance of the cumulative pH of a person’s diet, it is not at all contentious that the foods that are highest in minerals and are alkaline in pH are very good for you, e.g. most fruits and vegetables. So, regardless of the reasons, an abundance of those foods should be a highlight of any athlete’s diet.
Basically, as far as protein goes, an athlete should be free to eat most any source of protein that’s available to them and that they enjoy, because it’s a part of a balanced diet and will provide nutrients necessary for good nutrition and hard training, but for each serving of animal protein, it’s probably best to have at least one or two servings of fruit or vegetable.