diff --git a/src/Config/Environment.php b/src/Config/Environment.php
index b8f06e2309..68d201b8da 100644
--- a/src/Config/Environment.php
+++ b/src/Config/Environment.php
@@ -48,14 +48,24 @@ public function loadSiteAutoloader($root)
             return false;
         }
 
-        if (!$this->siteLoader) {
-            $this->siteLoader = require $autloadFilePath;
-            if ($this->siteLoader === true) {
-                // The autoloader was already required. Assume that Drush and Drupal share an autoloader per
-                // "Point autoload.php to the proper vendor directory" - https://www.drupal.org/node/2404989
-                $this->siteLoader = $this->loader;
-            }
+        if ($this->siteLoader) {
+            return $this->siteLoader;
+        }
+
+        $this->siteLoader = require $autloadFilePath;
+        if ($this->siteLoader === true) {
+            // The autoloader was already required. Assume that Drush and Drupal share an autoloader per
+            // "Point autoload.php to the proper vendor directory" - https://www.drupal.org/node/2404989
+            $this->siteLoader = $this->loader;
         }
+
+        // Ensure that the site's autoloader has highest priority. Usually,
+        // the first classloader registered gets the first shot at loading classes.
+        // We want Drupal's classloader to be used first when a class is loaded,
+        // and have Drush's classloader only be called as a fallback measure.
+        $this->siteLoader->unregister();
+        $this->siteLoader->register(true);
+
         return $this->siteLoader;
     }
 
diff --git a/src/Preflight/Preflight.php b/src/Preflight/Preflight.php
index c2762f39af..289f3b06da 100644
--- a/src/Preflight/Preflight.php
+++ b/src/Preflight/Preflight.php
@@ -189,10 +189,9 @@ protected function doRun($argv)
         $runner = new \Robo\Runner();
         $runner->registerCommandClasses($application, $commandClasses);
 
-        // TODO: Consider alternatives for injecting '$root' into the bootstrap manager.
-        // Also, maybe we should inject the DrupalFinder from this class instead.
+        // TODO: Consider alternatives for injecting DrupalFinder into the bootstrap manager.
         $bootstrapManager = $container->get('bootstrap.manager');
-        $bootstrapManager->locateRoot($root);
+        $bootstrapManager->setDrupalFinder($this->drupalFinder);
 
         // Run the Symfony Application
         // Predispatch: call a remote Drush command if applicable (via a 'pre-init' hook)