Assombalonga made his return for Forest during their match against Blackburn Rovers last night, which finished as a 1-1 draw.
The 23-year-old forward had been out since February 2015 with a knee injury, but he came on as a late substitute against Blackburn.
Assombalonga was given a great ovation by Forest fans as he came on, and Williams has hinted that the forward could feature again before the season is out.
“Getting him playing time is crucial and once he gets a pre-season under his belt you’ll have a fantastic footballer on your hands," Williams said.
“Britt worked hard on his rehab – all credit to the medical staff also. They’ve done really well getting him back and now he’s playing, so we’re grateful.
“We’ll see how Britt is tomorrow morning and assess how he is mentally and physically. Hopefully we can tease a bit more time out of him as the games go.”
Assombalonga has already stated that he is keen not to rush back, with Forest having very little to play for this season.
The Reds are destined to finish in the bottom half, but they are comfortably clear of relegation - meaning that the pressure is off.
Assombalonga has suggested that he does not want to risk his health by coming back too quickly, but his return to the side is a major boost.
Forest have struggled for goals in Assombalonga’s absence, and his comeback makes their attacking options look far stronger heading into next campaign.