The veteran Portuguese forward may want to depart United, as they are not in the Champions League.
Reports in the European press have linked him to Bayern Munich and Chelesa in recent weeks.
However, Brown hopes United hold on to Ronaldo for one more year.
He told Daily Express: "He's a different breed.
"We need to make signings regardless but keeping Ronaldo will be a big plus, just look at the goals he got us this year.
"You can say lots of things good, or bad, but ultimately he will score you goals as well.
"Obviously Cavani has gone. Pretty much every position we need to improve on and I'm pretty sure the manager will know that, he's been told that's absolutely fine and the funds are there to do that.
"Ultimately it's about whether we can get hold of these players and can we get them at the right price as well."