The keeper is working his way back from a calf blow.
Achterberg told Liverpoolfc.com: "Ali has been working with Dave [Rydings] in the gym a lot to improve the strength and try to keep the fitness levels up.
"In the last few days he has been out on the pitch to do some catching and footwork, as well as small passing drills.
"He has made steady progress, but it is difficult to say how long it will take or how it will progress.
"It needs time and we cannot look too far ahead, so we take each day as it comes, see how he feels, and then make progress from that."