The forward moved to Roots Hall in January to help the Shrimpers in their bid to retain their Football League status.
However, the task proved too much, with Southend's relegation from League Two confirmed yesterday.
Although the Essex outfit defeated Barrow, Scunthorpe's goalless draw with Bradford meant they are mathematically safe with one game remaining.
Hackett-Fairchild has played regularly for Southend, scoring one goal in 24 appearances. He featured for 70 minutes against Barrow before being substituted.
He’ll return to the south coast this summer ahead of the 2021-22 season, with the ex-Charlton man under contract for another year.
Former Pompey pair Greg Halford and Rickie Holmes are also part of the Shrimpers squad that’s gone down.
Meanwhile, Northampton's immediate relegation back to the fourth tier represents another blow in what's been a challenging season for Morris.
Having recovered from a hip injury that sidelined him for 14 months, the ex-Middlesbrough man started Pompey's opening five league games of the term before he jettisoned down the midfield pecking order.
After falling out of favour at PO4, Morris joined the Cobblers in January to try to keep them in League One.
However, Northampton lost 3-0 to promotion-chasing Blackpool yesterday, meaning they're cut four points adrift form 20th-placed Wigan with one game remaining.
Morris had been expecting to depart Fratton Park when his contract expires this summer when he was granted an exit by former manager Kenny Jackett.
However, Danny Cowley, who succeeded Jackett in March, admitted he's being open-minded when it comes to the midfielder's future.