The Mirror says West Brom, who forked out a £1.5m fee to land the 24-year-old, are paying half of his £52,000-a-week City pay packet and have an option to make the deal permanent.
Albion have so far been left frustrated, with much of Sinclair's stay spent on the treatment table after his recovery took longer than initially hoped.
He was available for selection for Saturday's clash with Norwich but failed to make the Baggies' 18-man squad - and is now under pressure to deliver in the second half of the campaign.
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