Nathan is back training with the club in a bid to win a deal. The Wales striker is available on a free after being released by Leicester City at the end of last season.
He scored for United in a 2-1 win at Coventry, but he was badly injured and his loan spell was cut short.
Sam Ellis has worked closely with Robbie at Burnley and Leeds are reportedly monitoring his situation at Birmingham City, where he only moved last January.
Mikael Forssell's permanent arrival from Chelsea means his chances are now more limited up front at St Andrews.
Meanwhile, Brazil striker Catanha, who is available on a free after being released by Kryla Sovetov, is also reportedly showing interest at Leeds.