diff --git a/packages/autoloader/src/class-plugins-handler.php b/packages/autoloader/src/class-plugins-handler.php
index acc0f0fdf83d3..1131b07ed8a69 100644
--- a/packages/autoloader/src/class-plugins-handler.php
+++ b/packages/autoloader/src/class-plugins-handler.php
@@ -156,44 +156,4 @@ private function get_plugins_activating_via_request() {
 	public function get_current_plugin_dir() {
 		return explode( '/', plugin_basename( __FILE__ ) )[0];
 	}
-
-	/**
-	 * Resets the autoloader after a plugin update.
-	 *
-	 * @param bool  $response   Installation response.
-	 * @param array $hook_extra Extra arguments passed to hooked filters.
-	 * @param array $result     Installation result data.
-	 *
-	 * @return bool The passed in $response param.
-	 */
-	public function reset_maps_after_update( $response, $hook_extra, $result ) {
-		global $jetpack_autoloader_latest_version;
-		global $jetpack_packages_classmap;
-
-		if ( isset( $hook_extra['plugin'] ) ) {
-			$plugin = $hook_extra['plugin'];
-
-			if ( ! $this->is_directory_plugin( $plugin ) ) {
-				// Single-file plugins don't use packages, so bail.
-				return $response;
-			}
-
-			if ( ! is_plugin_active( $plugin ) ) {
-				// The updated plugin isn't active, so bail.
-				return $response;
-			}
-
-			$plugin_path = trailingslashit( WP_PLUGIN_DIR ) . trailingslashit( explode( '/', $plugin )[0] );
-
-			if ( is_readable( $plugin_path . 'vendor/autoload_functions.php' ) ) {
-				// The plugin has a v2.x autoloader, so reset it.
-				$jetpack_autoloader_latest_version = null;
-				$jetpack_packages_classmap         = array();
-
-				require $plugin_path . 'vendor/autoload_packages.php';
-			}
-		}
-
-		return $response;
-	}
 }
diff --git a/packages/autoloader/src/functions.php b/packages/autoloader/src/functions.php
index 18a1f3f3f61fa..221397d4cfa87 100644
--- a/packages/autoloader/src/functions.php
+++ b/packages/autoloader/src/functions.php
@@ -90,7 +90,55 @@ function set_up_autoloader() {
 	if ( empty( $jetpack_packages_classmap ) && $current_autoloader_version === $jetpack_autoloader_latest_version ) {
 		enqueue_files( $plugins_handler, $version_selector );
 		$autoloader_handler->update_autoloader_chain();
-		add_filter( 'upgrader_post_install', array( $plugins_handler, 'reset_maps_after_update' ), 0, 3 );
+		add_filter( 'upgrader_post_install', __NAMESPACE__ . '\reset_maps_after_update', 0, 3 );
 	}
 }
 
+/**
+ * Resets the autoloader after a plugin update.
+ *
+ * @param bool  $response   Installation response.
+ * @param array $hook_extra Extra arguments passed to hooked filters.
+ * @param array $result     Installation result data.
+ *
+ * @return bool The passed in $response param.
+ */
+function reset_maps_after_update( $response, $hook_extra, $result ) {
+	global $jetpack_autoloader_latest_version;
+	global $jetpack_packages_classmap;
+
+	if ( isset( $hook_extra['plugin'] ) ) {
+		/*
+		 * $hook_extra['plugin'] is the path to the plugin file relative to the plugins directory:
+		 * https://core.trac.wordpress.org/browser/tags/5.4/src/wp-admin/includes/class-wp-upgrader.php#L701
+		 */
+		$plugin = $hook_extra['plugin'];
+
+		if ( false === strpos( $plugin, '/', 1 ) ) {
+			// Single-file plugins don't use packages, so bail.
+			return $response;
+		}
+
+		if ( ! is_plugin_active( $plugin ) ) {
+			// The updated plugin isn't active, so bail.
+			return $response;
+		}
+
+		/*
+		 * $plugin is the path to the plugin file relative to the plugins directory.
+		 * What if this plugin is not in the plugins directory, for example an mu plugin?
+		 */
+		$plugin_path = trailingslashit( WP_PLUGIN_DIR ) . trailingslashit( explode( '/', $plugin )[0] );
+
+		if ( is_readable( $plugin_path . 'vendor/autoload_functions.php' ) ) {
+			// The plugin has a v2.x autoloader, so reset it.
+			$jetpack_autoloader_latest_version = null;
+			$jetpack_packages_classmap         = array();
+
+			set_up_autoloader();
+		}
+	}
+
+	return $response;
+}
+