Access and modify Text within a prefab

I am making my first non-tutorial game in Unity, and it's going well in most ways. I am stuck however, trying to write to a Text within a prefab during an instance of that prefab. In my game, the player triggers a battle, which generates an instance of a battle environment. Within that environment a copy of the player and a monster are also instantiated from prefabs. Everything's great, the internal UI works, and even the UI childed to the characters is showing up. I can't modify it, though. I kept getting the "object not a reference to an instance" error, until I added an if statement to see if the variable I stored the Text object in was null (which is always is). Any idea where I'm going wrong? Thanks so much!

 using UnityEngine;
 using UnityEngine.UI;
 using System.Collections;
 using UnityEngine.SceneManagement;
 
 public class GameManager : MonoBehaviour {
 
     public int test;
 
     //Starting player health
     private static float MAX_HEALTH = 100f;
     public float playerHealth = MAX_HEALTH;
     private float healthRatio;
     public GameObject healthBar;
     //Placeholder starting monster health
     public float monsterHealth = 50;
     //To track choices in battle
     public int numAttacks = 0;
     public int numRun = 0;
 
     //For setting up and breaking down battles
     public GameObject battleParticles;
     public GameObject battleGround;
     public GameObject player;
     public GameObject playerImage;
     public Text playerText;
     public GameObject mainCam;
     public GameObject monster;
     public Text monsterText;
     //For setting up and breaking down battles
     private GameObject clonePlayer=null;
     [HideInInspector] public GameObject cloneBattle=null;
     private GameObject cloneMonster=null;
     private Vector3 playerOrigin;
 
     //Allow other classes to manipulate health/score
     public static GameManager instance = null;
 
     //Sets up singular instance of GameManager for other classses to use
     void Awake (){
         if (instance == null)
             instance = this;
         else if (instance != this)
             Destroy (gameObject);
     }
 
     //Freeze the player, set up battle, start battle
     public void startBattle(){
         player.GetComponent<PlayerController> ().enabled = false;
         Instantiate (battleParticles, player.transform.position, Quaternion.identity);
         Invoke("initializeBattle", 2f);
     }
 
     //Start battle
     public void initializeBattle(){
         if (cloneBattle == null) {
             cloneBattle = Instantiate (battleGround, new Vector3 (0, 50, 0), Quaternion.identity) as GameObject;
             clonePlayer = Instantiate (playerImage, new Vector3 (-4f, 51f, 0f), Quaternion.identity) as GameObject;
             cloneMonster = Instantiate (monster, new Vector3 (4f, 51f, 0f), Quaternion.identity) as GameObject;
             playerText = clonePlayer.transform.GetChild (0).gameObject.GetComponent<Canvas> ().GetComponent<Text> ();
             monsterText = cloneMonster.transform.GetChild (0).gameObject.GetComponent<Canvas>().GetComponent<Text> ();
         }
     }
         
 
     //End battle, unfreeze player
     public void endBattle(){
         Destroy (cloneMonster);
         Destroy (clonePlayer);
         Destroy (cloneBattle);
         player.GetComponent<PlayerController> ().enabled = true;
     }
 
     //Functions called to reduce health if player/monster is injured
     public void playerInjury(float lostHealth){
         if (playerText) {
             playerText.text = "-" + lostHealth.ToString () + "!";
         }
         playerHealth -= lostHealth;
         healthRatio = (playerHealth / MAX_HEALTH);
         healthBar.GetComponent<RectTransform> ().localScale = new Vector3(healthRatio, healthBar.GetComponent<RectTransform> ().localScale.y, healthBar.GetComponent<RectTransform> ().localScale.z); 
         //Placeholder
         Debug.Log ("Player: " + playerHealth);
         if (playerHealth <= 0f) {
                 SceneManager.LoadScene ("Lose_Screen");
         }
     }
     public void monsterInjury(float lostHealth){
         if (monsterText) {
             monsterText.text = "-" + lostHealth.ToString () + "!";
         }
         monsterHealth -= lostHealth;
         //Placeholder
         Debug.Log ("Monster: " + monsterHealth);
         if (monsterHealth <= 0f) {
             endBattle ();
         }
     }
 
     //Functions called to track player choices in battle
     public void playerAttack(){
         numAttacks++;
         Debug.Log ("Attacked: " + numAttacks);
     }
     public void playerRun(){
         numRun++;
         Debug.Log ("Ran: " + numRun);
     }
 
 }

In the above code, I have assembled a Game Manager which works marvelously well in all ways except for one. In each of these places:
if (monsterText) {
monsterText.text = "-" + lostHealth.ToString () + "!";
}
if (playerText) {
playerText.text = "-" + lostHealth.ToString () + "!";
}

The if statement is always false. monsterText and playerText are always null (or simply throw the "object reference not set to an instance" error), meaning something fails in their assignment, which occurs here:

 clonePlayer = Instantiate (playerImage, new Vector3 (-4f, 51f, 0f), Quaternion.identity) as GameObject;
 cloneMonster = Instantiate (monster, new Vector3 (4f, 51f, 0f), Quaternion.identity) as GameObject;
 playerText = clonePlayer.transform.GetChild (0).gameObject.GetComponent<Canvas> ().GetComponent<Text> ();
 monsterText = cloneMonster.transform.GetChild (0).gameObject.GetComponent<Canvas>().GetComponent<Text> ();

I've tried breaking that statement up and using temporary variables, but it's never paid off. I would like to update the text during each battle instance to show the value of blows landing as they happen. I thought setting and calling the variables after creating the instance would be enough, but Unity is still telling me the object is not set to an instance.
What am I missing?