The Holland defender has been missing since December due to a back injury.
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"It's difficult to say, just because it's taken that much time," Carrick said ahead of tonight's clash with Bournemouth.
"With backs sometimes you think it's all right and all of a sudden it's not quite right.
"We're just being patient and working through it. We'll give him time and try to get him back as quickly as we can, but it's really difficult to say."
Asked if surgery will be required, Carrick also said: "I don't want to get into that at this stage. We'll try and do whatever we can to help and we're working through that now.
"I'm not trying to hide anything. He's improving. It's just not as quick as we all hope."