Derby boss Wayne Rooney has confirmed that Mengi will return to United after suffering a season-ending injury.
"Mengi will miss the rest of the season and go back to Manchester United from today," Rooney confirmed.
"(He's been) fantastic for us. I've seen him grow with each game. We will keep in contact with United and Teden about next season.
"The injury isn't too serious but he won't recover in time to play again."
Rooney admitted earlier this month that Derby have an agreement in principle that could see Mengi return to Pride Park next season.