Can the MVC Controller method be mocked using the NSubstitute

I need to test and mock a method found inside a controller. Is is possible to mock a method inside the controller, without implementing an interface using NSubstitute framework.

Here is my Controller page code.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.Mvc;

namespace Syncfusion.Cluster.Manager
{
    public class HomeController : Controller
    {
        // GET: Home
        public ActionResult Index()
        {
            int result = Sum(7, 7);
            return View();
        }

        public virtual int Sum(int a, int b)
        {
            return 0;
        }
    }
}

Here is my Test code and the Sum always found to be '0' inside the controller while I debugged it. The Sum function inside the controller is not get overrides.

using System;
using NUnit.Framework;
using Syncfusion.Cluster.Manager;
using Syncfusion.Cluster.Manager.Base.Classes.Models.SecurityBase;
using NSubstitute;
using BaseProject;
using System.Web.Mvc;
using System.Web.Http;

namespace NSubstituteControllerSupport
{
    [TestFixture]
    public class UnitTest1
    {
        [Test]
        public void TestMethod1()
        {
            var controller = new HomeController();           
            //var actionMethod = Substitute.For<HomeController>();
            //actionMethod.Sum(Arg.Any<int>(), Arg.Any<int>()).Returns(14);
            //var actual = controller.Index();

            var validation = Substitute.ForPartsOf<HomeController>();
            validation.When(x => x.Sum(a: Arg.Is(7), b: Arg.Is(7))).DoNotCallBase();
            validation.Sum(7, 7).Returns(14);
            var actuals = controller.Index();

        }
}

Here is my testing code for the Class Library project I successfully mocked a method without implementing an interface.

        [Test]
        public void TestMethod2()
        {
            #region variableDecleration
            var adHost = new ActiveDirectoryHost();
            adHost.AdPath = @"C:\User\Security\1.0.0.0";
            adHost.CnNames = "CN=USERS";
            adHost.DomainName = "USER.COM";
            adHost.DomainPath = "DC=USER,DC=COM";
            adHost.Fqn = "HOSTNAME.SYNCFUSION.COM.SYNCFUSION.COM";
            adHost.HostName = "hostname.syncfusion.com";
            adHost.OuPath = "OU=NewSecur";
            adHost.SuperGroupName = "usergroup_1";
            adHost.IpAddress = "xxx.xx.xx.x";
            var adUserName = "username";
            var adPassword = "password";
            #endregion variableDecleration

            var validation = Substitute.ForPartsOf<ValidationHandler>();
            validation.GetUserGroupList(userName: Arg.Is(adUserName ), recursive: Arg.Is(false), adHost: Arg.Is(adHost), adUsername: Arg.Is(adUserName), adPassword: Arg.Is(adPassword)).Returns("usergroup_1,usergroup_2");
            var isUserInGroup = validation.IsUserMemberOfGroup(adUsername, "usergroup_1", adHost, adUserName, adPassword);
            Assert.That(isUserInGroup, Is.EqualTo(true));
}