The Argentine international had been linked with the Red Devils during the transfer window before deciding to stay with the Serie A club.
Following reports that he rejected the chance to join United, Romero insists that a switch to Old Trafford was always unlikely.
"I had two or three possibilities but none of them were positively concluded," he told Il Secolo XIX. "I didn't ask for more money to go to Benfica and I also read that I turned down United.
"But, in your opinion, would someone be able to refuse an offer from one of the world's top clubs? I want to show my value here, so I'll be able to find my space."
Romero, who spent last season on loan with Monaco, joined Sampdoria in 2011.