how to implement a Default Item property with two Parameters

Formally, this is the indexed property. This is how:

class PlayerSet {
 
   //...

   internal Player this[int index, int jerseyNumber] {
      get {
         return FindPlayer(index, jerseyNumber); // but it's up to you what you return; it could be contradictory
      } //get Default
   } //Default

} //class PlayerSet

However, it's questionable if the whole idea makes sense. Usually, you can find one single item by index, or zero items, and one single item by key, or zero items. The key and index may contradict. Perhaps you should better use two different property, any this was the reason of many questions, because many people failed to figure out the syntax. One of the problems is identical types of the index and the key. This is one of the ways:

class JerseyNumber { // just the wrapper, to create different type; you can think of something different
   internal JerseyNumber(int index) { this.Index = index; }
   internal int Index { get; private set; }
} //class JerseyNumber

interface IKeyedPlayerSet {
    Player this[JerseyNumber jerseyNumber] { get; }
} //interface IKeyedPlayerSet

class PlayerSet : IKeyedPlayerSet {

    public Player this[JerseyNumber jerseyNumber] {
       return FindPlayerByKey(jerseyNumber.Index); // by integer key
    } //IKeyedPlayerSet.this

    internal Player this[int index] { /* access by index */ }

    //...
   
} //class PlayerSet

These are just the ideas. I don't know some important detail of your implementation of player collection, so something in my samples may be not adequate to it, but I hope you still can figure out what to do.

—SA