The midfielder has been linked with Tottenham and Crystal Palace this month.
Hellberg said: “He’s everything you want in a player. An unbelievable football player, and he wants to do everything for Middlesbrough. He’s always happy, always smiling, always working hard.
“He’s too good to be here — if you can say that in a positive way — as could be top in most of the Premier League teams, because that’s the quality he has. And he still works together with everyone else to make everyone happy here.
“You cannot be more proud than when you see how he fights for everyone else, how every training he helps everyone.
“That is what you want in a captain and as a leader and as a player for the club. He’s amazing in every way.
“He shows up every day with all the energy he has, a smile and works so hard. He really deserved that goal, an unbelievable finish. He’s top at everything he does.”