The Forest academy graduates, 20 and 23 years old respectively, have signed permanent deals with the Premier League club but will remain at The City Ground on season-long loans as part of the agreement.
Centre back Lascelles rose to prominence last season after making a handful of senior appearances in previous campaigns. His athleticism, stature and aerial prowess make him a menacing figure in The Reds’ back four.
Goalkeeper Darlow took the number one mantle from Lee Camp midway through the 2012-13 season and has held on to the shirt ever since with an array of impressive performances.