
Agyemang underwent the knife last week to repair a ruptured Achilles that will keep him sidelined until Christmas.
The injury has not only ruled him out of Derby's play-off push but also the World Cup this summer when he stood a good chance of making the USA squad.
Eustace said Agyemang felt "very low" in the aftermath of the injury but is looking forward to his return and says it will feel like a January signing.
"He's had his surgery probably about a week ago now, so it's going to be a slow, slow process for him," he said.
"We're all right behind him and making sure he's in the best hands. I don't think he'll be back now till Christmas, which is obviously a huge blow for us, but when he's back, it'll be like signing a fantastic player in January and hopefully we'll be in a good position where he can really impact us.
"It really hit him at the first because it's not just missing the playoff push with us but also missing the World Cup, which is a huge thing for him.
"Obviously to represent your country is an amazing achievement and he's come from nowhere really, at the start of the season to get to a really strong position now.
"From a mentality point of view, he's been very low and, but he has started to come in now for the last couple of days.
"He has a big smile back on his face and we're right behind him."
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