Ferguson said the flexibility to the partnership was key but that the pair had yet to hit the highest level together, even if there were flashes in the 2-1 win over the Gunners at the weekend.
Ferguson said: "There's flexibility there and the right type of movement. They also both have the courage to play in these positions. Robin's not far away in that respect.
"They're both mobile players and good footballers with a great desire to play and win. I don't think you saw the best of them as a partnership against Arsenal."