Celtic striker Oh Hyeon-gyu could be out for up to six weeks with a calf problem.

The 22-year-old South Korea international originally picked up the injury in James Forrest’s testimonial match against Athletic Bilbao but did not declare it before taking his place on the bench for the cinch Premiership opener against Ross County at Parkhead on Saturday.

“Oh will probably be out for a few weeks,” said Hoops boss Rodgers, who refused to elaborate on Celtic’s interest in Swedish centre-back Gustaf Lagerbielke, which had been revealed by Elfsborg manager Stefan Andreasson.

“He hurt his calf against Athletic Bilbao but no one knew and he trained on.

“He was involved in the game at the weekend and then he trained on the Sunday and afterwards he spoke to the medical team to say he felt his calf and it has ended up that he has a calf problem which might put him out for four to six weeks. So that’s where he is at.

“Apart from that, the injured guys are doing really well.

“Ali Johnston is back on the grass, running, working, he is increasing his intensity so that is really good news for us.

“Marco Tilio is also now out on the grass and moving well so we expect him to be another month or so away.”

Swedish central defender Carl Starfelt has been linked with a move to Celta Vigo after Rodgers confirmed at the weekend he was keen to move on, but the manager said: “It is progressing but nothing definitive to add on that.”

Asked if he could say reveal anything on Lagerbielke, the Northern Irishman said: “No. Clearly if we lose a centre-half I want to bring in another one, but I have got nothing to add.

“There’s probably be a few other names that will be mixed in there and if I spoke about every one we might be here for a little while.

“There is a number that we are looking at, but nothing definitive.

“There are other areas I would like to improve on.”

Meanwhile, in the week that some of Scotland’s other representatives are trying to qualify for the group stages of their respective European tournaments, Scottish champions Celtic have the advantage of having already booked their place in the Champions League, which starts in September.

Rodgers, however, is bracing himself for the toughest of tests and insists there is “absolutely no doubt” that the standard of European club football’s premier competition increases year on year.

He said: “It is always a challenge. By the time we get to mid-September this group will be even better both in terms of hopefully depth and affinity of how we are going to play and work and the adaptation to the changes.

“I like to develop teams and improve players and with five weeks or so to go before the first game I would like to think we are in a better place, but whatever squad we have here it is always going to be a huge challenge.

“I’m also a realist. I like the supporters to dream, I like to dream of doing great things with Celtic, but I am also a realist.

“When you look at the finances involved and the players at that level then it is an absolutely huge challenge.

“For a club like ourselves, in comparison to those clubs economically, it is always going to be that. But it is a competition that we want to be competitive in.

“I know that when we arrive into it we will give it our very best shot and see where it takes it us.”.