Rovers have had two bids for the Us hitman turned down already this summer and McGhee says he is still on their list as a new striker remains top priority for the Pirates.
"He has a record at this level and that's important because I want to start with a team that has a record," he told BBC Radio Bristol.
"If I look at my starting line-up on the first day of the season and it's got a history of 40 goals a season then we are in trouble.
"I want to start with a team that's got 65 goals as an average. Then we'll have a chance."
Constable netted 11 goals for the Us last season and has two years to run on his contract at the Kassam Stadium.