Manager Steve Bruce initially suggested Koren (fractured foot) and Chester (hamstring) would be ruled out for eight weeks each, a lay-off that would see both men absent until at least the middle of November.
But the club's head of medical services, Rob Price, offered an upbeat assessment on the duo.
"With injuries, it isn't always black and white and you have to see how people progress," Price told the Tigers' official website.
"When we say it could be eight weeks, that doesn't necessarily mean that it will be eight weeks because we always hope we can get them back a little bit quicker than that.
"Robert is doing particularly well with his rehab and we're hopeful that within the next three weeks he could be back out there with the team.
"Robert hasn't had a day off since he got the injury and he's using this time as an opportunity to work on other parts of his body like his strength and his core. He's also helping out the other players during their training sessions and being one of our senior pros, he's also passing on plenty of advice.
"With James, he has some damage to one of his hamstring tendons and you have to really give that a couple of weeks to settle down for it to start to form some scar tissue before we can start loading him.
"But he's doing well in the early stages of his rehab and again, he's also working on some other areas. Hopefully in around 10 days we'll be able to start pushing him a little bit more."
- Sports & Recreation