The Burundi intrnational spent the second half of last season on loan at Celtic, where he scored four goals after battling for game-time at Sundowns.
He had netted on debut for The Brazilians, but then fell down the pecking order after the arrival of Leonardo Castro.
Celtic CEO Khumbulani Konco has confirmed that Razak is back with Siwelele.
“He is back with us on a season-long loan deal and comes as part of the Morena deal,” Konco tells KickOff.com.
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Meanwhile, Celtic have decided against signing Orlando Pirates striker Mwape Musonda.
Musonda – who spent the past season on loan at Lamontville Golden Arrows – is not part of the plans at Pirates this season despite still having another two years to run on his contract.