Ssewankambo was sold to Norwegian side Molde for an undisclosed fee on Thursday, a decision which prompted a reaction of disappointment from many Rams supporters on Twitter.
@Ssewankambo30 gutted you never got a chance, all the best for the future!— Mark Cordin (@Mark_Cordin) 3 March 2016
@Ssewankambo30 All the best for the future Isak, never let us down here at Derby, thought u had a great future @ DCFC, u will do great there— DJPritchett34 (@Davidjpritch34) 3 March 2016
But Wassall, who took the reins at the Ipro Stadium last month, has put forward his case.
"Isak wanted first-team football and we couldn't guarantee that," said the 47-year-old, as quoted by the Derby Telegraph.
"He had a year left of his contract, Molde put in a very good bid and I think it is best for both parties.
"Isak goes with our best wishes . He has been a model professional since he has been here."
Former Chelsea youngster Ssewankambo departs little over a year since his arrival from Dutch side NAC Breda.
The right-back appeared for the Rams' first team just once in that time, playing 70 minutes of August's Capital One Cup defeat at Portsmouth. He made the bench for January's FA Cup win at Hartlepool United, but didn't get on.
According to HITC Sources, Wassall's predecessor Paul Clement was sacked due to his failure to integrate youth team players, and Wassall has already taken steps to address this, despite the departure of Ssewankambo, with 20-year-old Jamie Hanson restored to his squad for Derby's past three matches.
Last month, the former Rams player recalled youngsters Farrend Rawson and Offrande Zanzala from their loans at Rotherham and Stevenage respectively.