The Frenchman was subbed by Cole Kpekawa on 71 minutes, and long before the 4-2 penalty shoot-out that decided the match in Hoops favour.
"Yeni is not fit yet; we need to work him and give him time," Hasselbaink explained.
"He hasn't worked as intensively pre-season at Metz as he's working now. But you saw the quality here and there, and hopefully we can bring that to a regular basis."
This is something of what we can expect when Ngbakoto is up to full speed.
Instead, Washington notched his first competitive goal for Rangers in the third minute of extra time - seven months after he made a £2m switch from Peterborough.
In fairness to Washington, he was also injured as well as given pre-season time off after his Euro 2016 exploits for Northern Ireland - but there is more to come, according to JFH.
"It was a boost to his confidence to score, because he suffered a little bit against Swindon, but he kept on going and when you do the chance will come - and it did," he added.