diff --git a/rules-tests/CodingStyle/Rector/FuncCall/FunctionFirstClassCallableRector/Fixture/namespaced_function.php.inc b/rules-tests/CodingStyle/Rector/FuncCall/FunctionFirstClassCallableRector/Fixture/namespaced_function.php.inc
new file mode 100644
index 00000000000..9a986a657b3
--- /dev/null
+++ b/rules-tests/CodingStyle/Rector/FuncCall/FunctionFirstClassCallableRector/Fixture/namespaced_function.php.inc
@@ -0,0 +1,29 @@
+<?php
+
+namespace Rector\Tests\CodingStyle\Rector\FuncCall\FunctionFirstClassCallableRector\Fixture;
+
+final class NamespacedFunction
+{
+    public function run(): array
+    {
+        // String callable: resolved from the global namespace at runtime -> works.
+        return array_map('Cake\Utility\Inflector::underscore', ['FooBar', 'BazQux']);
+    }
+}
+
+?>
+-----
+<?php
+
+namespace Rector\Tests\CodingStyle\Rector\FuncCall\FunctionFirstClassCallableRector\Fixture;
+
+final class NamespacedFunction
+{
+    public function run(): array
+    {
+        // String callable: resolved from the global namespace at runtime -> works.
+        return array_map(\Cake\Utility\Inflector::underscore(...), ['FooBar', 'BazQux']);
+    }
+}
+
+?>
diff --git a/rules/CodingStyle/Rector/FuncCall/FunctionFirstClassCallableRector.php b/rules/CodingStyle/Rector/FuncCall/FunctionFirstClassCallableRector.php
index 5a610bc3e15..8b98d55fcf0 100644
--- a/rules/CodingStyle/Rector/FuncCall/FunctionFirstClassCallableRector.php
+++ b/rules/CodingStyle/Rector/FuncCall/FunctionFirstClassCallableRector.php
@@ -8,6 +8,7 @@
 use PhpParser\Node\Arg;
 use PhpParser\Node\Expr\FuncCall;
 use PhpParser\Node\Name;
+use PhpParser\Node\Name\FullyQualified;
 use PhpParser\Node\Scalar\String_;
 use PhpParser\Node\VariadicPlaceholder;
 use Rector\Rector\AbstractRector;
@@ -101,7 +102,12 @@ public function refactor(Node $node): ?FuncCall
             }
 
             $node->args[$key] = new Arg(
-                new FuncCall(new Name($arg->value->value), [new VariadicPlaceholder()]),
+                new FuncCall(
+                    str_contains($arg->value->value, '\\')
+                        ? new FullyQualified($arg->value->value)
+                        : new Name($arg->value->value),
+                    [new VariadicPlaceholder()]
+                ),
                 name: $arg->name
             );
             $hasChanged = true;