Koren was released by West Brom at the end of last season.
Former England striker Regis said: "England will find Koren is a danger.
"He's a confident player who likes to get on the ball and knows where the goal is. Probably 90 per cent of the players at the World Cup are looking to impress so they can move on to a bigger club.
"Koren's got a great football brain and did a great job for West Brom.
"He doesn't have a club and if he plays well against England with so many people from the Premier League watching, then there will be plenty of interest in him.
"They'll be looking at a player they can sign for free - it's a no-brainer."