Mock an object created in the middle of a method

I know that creating an instance of a Class in the middle of a method it's a bad practice since it makes code hard to test. But I can't refactor the code, so I need to find a way to mock an Object created with new in the middle of a method under test.

Used Frameworks: PHPUnit, Mockery, WP_Mock

Example: Here I need to mock the get_second_string() method from the instance of the class ExternalClass

Class MyClass {

    function methodUnderTest($string) {
        $objToMock = new ExternalClass();
        $second_string = $objToMock->get_second_string();
        $final_string = $string . $second_string;
        return $final_string;
    }
}
Class TestMyClass extends PHPUnit_Framework_TestCase {
    public function setUp() {
    }
    public function tearDown() {
    }

    public function test_methodUnderTest() {
        $externalObject = $this->getMockBuilder('ExternalClass')
                               ->setMethods(['get_second_string'])
                               ->getMock;
        $externalObject->expects($this->once())
                       ->method('get_second_string')
                       ->willReturn(' two');
        $testObj = new MyClass();
        $this->assertEquals('one two', $testObj->methodUnderTest('one');
    }
}