diff --git a/core/includes/install.inc b/core/includes/install.inc
index bf88901..aa8d049 100644
--- a/core/includes/install.inc
+++ b/core/includes/install.inc
@@ -1001,7 +1001,7 @@ function drupal_check_module($module) {
   // We can't use the module handler here because it only gives us enabled
   // extensions.
   $extension = new Extension(DRUPAL_ROOT, 'module', drupal_get_filename('module', $module));
-  $requirements = array_merge($requirements, $dependency_requirements->extensionComposerRequirements($extension));
+  $requirements = array_merge($requirements, $dependency_requirements->buildRequirements([$extension]));
 
   if (drupal_requirements_severity($requirements) == REQUIREMENT_ERROR) {
     // Print any error messages
diff --git a/core/lib/Drupal/Core/Composer/ExtensionDependencyRequirements.php b/core/lib/Drupal/Core/Composer/ExtensionDependencyRequirements.php
index 00211f1..fa5eecb 100644
--- a/core/lib/Drupal/Core/Composer/ExtensionDependencyRequirements.php
+++ b/core/lib/Drupal/Core/Composer/ExtensionDependencyRequirements.php
@@ -40,34 +40,42 @@ public function __construct(ExtensionDependencyChecker $dependency_checker, Tran
   }
 
   /**
-   * Determines whether Composer-based dependencies are still required.
+   * Builds the Composer requirements for extensions.
    *
-   * @param \Drupal\Core\Extension\Extension $extension
-   *   Extension to check.
+   * @param \Drupal\Core\Extension\Extension[] $extensions
+   *   The extensions to build the requirements for.
    *
    * @return array[]
    *   Requirements array, suitable for hook_requirements().
    */
-  public function extensionComposerRequirements(Extension $extension) {
+  public function buildRequirements(array $extensions) {
     $requirements = [
       'composer_dependencies' => [
         'title' => $this->t('Composer dependencies'),
       ],
     ];
-    if (!$this->dependencyChecker->dependenciesAreMet($extension)) {
-      $requirements['composer_dependencies'] = [
-        'description' => $this->t('Not all the modules have their Composer dependencies installed yet. Find more information on <a href="@handbook">this handbook page</a>.', [
-          '@handbook' => 'https://www.drupal.org/node/2627292'
+    $unmet_dependencies = FALSE;
+    foreach ($extensions as $extension) {
+      if (!$this->dependencyChecker->dependenciesAreMet($extension)) {
+        $unmet_dependencies = TRUE;
+        break;
+      }
+    }
+    if ($unmet_dependencies) {
+      $requirements['composer_dependencies'] += [
+        'description' => $this->t('Some modules require Composer dependencies. Read the <a href="@documentation">documentation</a> on how to install them.', [
+          '@documentation' => 'https://www.drupal.org/node/2627292'
         ]),
         'severity' => REQUIREMENT_ERROR,
       ];
     }
     else {
-      $requirements['composer_dependencies'] = [
+      $requirements['composer_dependencies'] += [
         'description' => $this->t('All Composer dependencies have been installed.'),
         'severity' => REQUIREMENT_OK,
       ];
     }
+
     return $requirements;
   }
 
diff --git a/core/modules/system/system.install b/core/modules/system/system.install
index afa5892..9e21ff2 100644
--- a/core/modules/system/system.install
+++ b/core/modules/system/system.install
@@ -7,6 +7,8 @@
 
 use Drupal\Component\Utility\Crypt;
 use Drupal\Component\Utility\Environment;
+use Drupal\Core\Composer\ExtensionDependencyChecker;
+use Drupal\Core\Composer\ExtensionDependencyRequirements;
 use Drupal\Core\Url;
 use Drupal\Core\Database\Database;
 use Drupal\Core\DrupalKernel;
@@ -239,6 +241,14 @@ function system_requirements($phase) {
     $requirements['php_extensions']['value'] = t('Enabled');
   }
 
+  // Check Composer dependencies during runtime, because drupal_check_module()
+  // does it when installing modules already.
+  if ($phase == 'runtime') {
+    $dependency_checker = new ExtensionDependencyChecker(DRUPAL_ROOT);
+    $dependency_requirements = new ExtensionDependencyRequirements($dependency_checker, \Drupal::translation());
+    $requirements = array_merge($requirements, $dependency_requirements->buildRequirements(\Drupal::moduleHandler()->getModuleList()));
+  }
+
   if ($phase == 'install' || $phase == 'runtime') {
     // Check to see if OPcache is installed.
     $opcache_enabled = (function_exists('opcache_get_status') && opcache_get_status()['opcache_enabled']);
diff --git a/core/tests/Drupal/Tests/Core/Composer/ExtensionDependencyRequirementsTest.php b/core/tests/Drupal/Tests/Core/Composer/ExtensionDependencyRequirementsTest.php
index e21b91b..1960315 100644
--- a/core/tests/Drupal/Tests/Core/Composer/ExtensionDependencyRequirementsTest.php
+++ b/core/tests/Drupal/Tests/Core/Composer/ExtensionDependencyRequirementsTest.php
@@ -21,14 +21,14 @@
 class ExtensionDependencyRequirementsTest extends KernelTestBase {
 
   /**
-   * Provides data to self::testDependenciesAreMet().
+   * Provides data to self::testBuildRequirements().
    *
    * @return array[]
    *   Every item is an array with the following items:
    *   - One of the REQUIREMENT_* constants.
    *   - A boolean TRUE if dependencies are met, FALSE otherwise.
    */
-  public function providerExtensionComposerRequirements() {
+  public function providerBuildRequirements() {
     // We cannot use any of the REQUIREMENT_* constants here, because providers
     // are run before environments are booted.
     return [
@@ -40,16 +40,16 @@ public function providerExtensionComposerRequirements() {
   }
 
   /**
-   * @covers ::extensionComposerRequirements
+   * @covers ::buildRequirements
    *
-   * @dataProvider providerExtensionComposerRequirements
+   * @dataProvider providerBuildRequirements
    *
    * @param int $expected_severity
    *   One of the REQUIREMENT_* constants.
    * @param bool $dependencies_met
    *   Whether the extension's dependencies have been met.
    */
-  public function testExtensionComposerRequirements($expected_severity, $dependencies_met) {
+  public function testBuildRequirements($expected_severity, $dependencies_met) {
     $dependency_checker = $this->getMockBuilder(ExtensionDependencyChecker::class)
       ->disableOriginalConstructor()
       ->getMock();
@@ -65,7 +65,7 @@ public function testExtensionComposerRequirements($expected_severity, $dependenc
 
     $sut = new ExtensionDependencyRequirements($dependency_checker, $string_translation);
 
-    $requirements = $sut->extensionComposerRequirements($extension);
+    $requirements = $sut->buildRequirements([$extension]);
     $this->assertCount(1, $requirements);
     $this->assertArrayHasKey('composer_dependencies', $requirements);
     $this->assertArrayHasKey('severity', $requirements['composer_dependencies']);