Nelsen said: "I think Brad is quite comfortably the best in the Premier League.
"Some of the saves he made against United were unbelievable. Two were just world class, and no other keeper would have made them.
Nelsen believes the former United States international is that good, it is remarkable Rovers have been able to keep him out of the clutches of the big four' for so long.
"If he'd been to a smaller club first and then later on went there, I think he would have had a fantastic career because he could play for any club in the world - he's that good.
"But I'm just so glad he's behind us, and he's a great professional as well.
"I think he will be remembered at Blackburn for a long time."