Huddersfield Town defender Jon Gorenc Stankovic will go to London next week for an operation on a knee injury he picked up on international duty last week.
The centre-back injured his anterior cruciate ligament (ACL) while training with Slovenia's Under 21 side and will be out for up to nine months.
It is a big blow for Town, who already have a long injury list heading into the final nine matches of the Championship season.
Kasey Palmer, Izzy Brown, Sean Scannell, Harry Bunn and Jonathan Hogg are working their way back to fitness, but are all likely to miss the Burton match this weekend.
And Stankovic will likely be out until the festive period later this year.
On the 21-year-old's injury, Wagner said: "I think he is okay.
"He has an ACL [injury]. He will have surgery on Wednesday next week in London and then, like always after such an injury, you expect the player out for six to nine months.
"Of course it is an injury which unfortunately happens, but I've spoken with him - he's still positive.
"I think now he likes to have his surgery next week and he knows exactly that he did a fantastic job in my opinion in this season so far if you know that he is 19 now 20.
"He helped us a lot this season and now he has to make the next step a little bit later than he expected.
"What he will do next season he will now make it in the middle of next season - so the advantage for him as I said to him is that in England you don't have a winter break so he can immediately start around Christmas to help and to play football again.
"We will be happy when he is back but now all his energy and our thoughts have to go to him in his surgery and his rehabilition."