The Brazil striker has arrived from Wolves, joining last month for a fee of almost £65m.
Speaking to Tribalfootball.com, Parker said: "He will definitely make them better, but I don’t know how many points he will put on the table. He’s so energetic it’s incredible.
"He will be the quickest player over any game as United don’t have quick players, and I reckon United are the slowest team I have watched other than West Ham.
"So he’s gonna make and effort and an impact, and wears his heart on his sleeve when he plays."
Good Old Trafford form
Parker admits Cunha already has shown he can perform at Old Trafford.
He also stated, "I watched him at Old Trafford when they played the season before last and in that first 45 minutes, he could have had a hattrick!
"He was absolutely incredible; incredible what he did. So, he will add something but I don’t know how his game is gonna be and he was playing in a good Wolves side at the end of the year.
"He wanted to come to United though and that makes all the difference. I think that gets the fans behind him who will give him time because he wanted to come to United. ‘No-one wants to join United because they’re not in Europe’ – what a load of b*llocks!"
- Paul Parker was speaking exclusively to Tribalfootball via the home of betting