The 17-year-old midfielder looked set to join the Champions of Africa, after leaving an impression on the Premiership side’s technical team during a play-off with Kampala Capital City Authority for this season’s CAF Champions League.
Downs narrowly beat KCCA 3-2 over the two legs but the display of Allen prompted Mosimane’s side to make a move for the teenager.
However, his agent revealed they opted for the Eredivisie giants after discovering Mosimane had plans to use Okello within the development squad.
“The deal was never going to happen because Mosimane let it slip that he would take Okello to the juniors,” his agent told The Citizen.
“Sundowns’ development is not known for producing any remarkable players and that was a major turn off. Okello is seen as the best thing to come out of Uganda in recent years and everyone wants him to become the best.
“He is now going to Ajax Amsterdam and they believe he will be properly developed into a fine player there.”