Onyango announced his retirement on April 12, ending his 16-year international career.
Williamson worked with the goalkeeper during his time as Cranes coach from 2008-2013.
“Onyango is a quality goalkeeper who has been very influential for the national team,” he was quoted saying.
“He will be a very big loss; he has great experience and a very good goalkeeper; his presence will be missed,” he added.
Williamson believes it’s a chance for other goalkeepers
Meanwhile, Williamson believes this is a blessing in disguise for other goalkeepers.
Other goalkeepers must show what they have to offer for the East African heavyweights according to the Scottish coach.
“It is a fact, getting an Onyango replacement will not be easy because we all know the quality of the player,” the former Harambee Stars coach continued.
“But we have goalkeepers who have been yearning for an opportunity to prove their worth.”
“So Onyango’s retirement provides a massive chance for them to develop,” he further added.
Since leaving Cranes, Williamson has coached Gor Mahia and Kenya national team.