The English coach confirmed at a press conference on Friday that he had almost expected the injury after three away games within six days.
Monday against OB, Thursday in Farum against FC Nordsjaelland, and Sunday in Silkeborg.
"Last week's program had not been fair to any team, but it hit us. I think it was almost inevitable that there would be an injury victim (for the programme, ed.), and unfortunately, it was Vallys."
"It's not a long-term injury, but it's an injury that hasn't gone away. But we'll work hard to get him ready as soon as possible, and then I'll focus on those who are ready", said Cooper
The Brondby coach also gave an update on some of the other players who have struggled with injuries recently.
Sean Klaiber will be out for a little while longer and won't be ready for Sunday's game either.
Sho Fukuda is back in training after a tough bout of illness, but it may be a tad too early to throw him into action this weekend.
Brondby meets FC Nordsjaelland on Sunday at 20:00.