Mengi, 18, will be at Pride Park on loan until the end of the season and has represented England at under-18 level.
Rooney was keen to bring in another defender in January following Mike te Wierik’s unsuccessful spell at the club after moving from Eredivisie side Groningen in the summer before returning there permanently earlier this month.
Teden Mengi is an exciting signing having broken into Manchester United’s first-team set-up
Wayne Rooney’s influence was significant in bringing Mengi to Derby from his former club
Mengi has been training regularly with United’s first team and made his debut for the Red Devils last August, coming on for Timothy Fosu-Mensah in the 84th minute as Ole Gunnar Solskjaer’s side beat LASK in their round-of-32, second-leg Europa League clash.
Speaking to Derby’s media team after his signing was confirmed, Mengi said: ‘I’m really excited and I’m buzzing to be here.
‘When I first heard Derby County were interested, it was a no-brainer for me to be in the Championship against good opponents and to be playing men’s football. I am really happy to be here.
‘I think I did well in the Under-23s last season so I think I need a new challenge. I am a fighter with a willingness to defend and a ball-playing centre back and vocal as well. I am ready to kick on and this is a brilliant opportunity for me.’
The Rooney factor was undoubtedly a significant pull for Mengi.
‘He has been brilliant,’ the teenager said. ‘As soon as I found out Derby were interested, Wayne was on the phone.
The 18-year-old made his Red Devils debut against LASK in the Europa League last August
‘He was telling me what the club is about and what he wants from me. He has been brilliant with me so far.’
Mengi could make his debut as early as tomorrow when the 18th-placed Rams travel to 22nd-placed Rotherham as Rooney’s men continue their push away from the relegation zone and into the calmer waters of mid-table.