Mascherano said: "The arrival of Carlos is a double-edged sword. I am happy for him but because he has gone to a rival club I am worried about him because he can do great things.
"Carlos is an explosive forward and is able to unbalance any defence. He is a big danger to everyone in this league."
Mascherano said both he and his international team-mate had to be strong to withstand their torrid first few months in England.
"Those early months (at West Ham) were terrible for us because the coach did not have confidence in us but now we are seeing what Carlos can do," he said.
"He needed that chance, just as I did, to adjust to England and to start playing regularly. That builds up your confidence and belief in your ability to succeed."
Now, with Tevez at Old Trafford, Mascherano is expecting a healthy rivalry.