Billy Vunipola faces a race against time to be fit for the Rugby World Cup after undergoing a minor operation on his injured knee.READ | Rugby World Cup: Wallabies name Hooper, Slipper co-captainsThe England forward remains with a squad named by head coach Steve Borthwick for their third World Cup training camp in south-west London."The medical team are positive about my prospects and I understand exactly what needs to be done over the next few weeks," said Vunipola.Will Joseph and Beno Obano are the only two players cut from the squad by Borthwick as players from Premiership finalists Sale and Saracens are still not included.England men's head coach Steve Borthwick has selected a 36-player training squad ??Players from Premiership finalists Sale Sharks and Saracens are not available for selection.@o2 | #WearTheRoseBilly Vunipola underwent a minor surgical procedure on his injured knee on Friday and will continue his rehabilitation, aiming to be available for Rugby World Cup selection in August"I'm pleased with the progress that's been made over the last two weeks and we look forward to another productive training camp," said Borthwick."There's still a long way to go until we are ready for a World Cup but each day we take another step forward."Ollie Chessum, Luke Cowan-Dickie, Ollie Lawrence and Jack Walker all remain with their teammates as they continue their rehabilitation from injury.England will take on Wales in Cardiff on 5 August, with Borthwick due to name his 33-strong squad for the World Cup in France two days later.