The Red Devils signed him from Crewe Alexandra in 2012, and the teenager went onto score on his debut against Wigan.
He has failed to kick on since then, and faces an important season in his development after being overlooked by Louis van Gaal last season.
What has gone wrong for Powell?
The biggest hindrance Powell has faced at United is arriving at a period of instability for the club.
He was one of Sir Alex Ferguson's last signings for the Red Devils and played one season under the Scotsman before he announced his retirement.
David Moyes was less enthused, sending him to Wigan Athletic on loan where he performed strongly, reaching double figures in goals scored in all competitions, but 2014/15 was poor, playing just four first team games for United, and making only nine appearances for the under-21s.
United's managerial changes have led to spending sprees, increasing competition for places. With Powell not firing on all cylinders, a loan move appears his best option of getting himself noticed again.
The 21-year-old should have been at the England under-21 tournament this summer, and needs to use his lack of progression as motivation to kick on this coming season - whether Forest are the best club for him to do so at remains open to debate.