A replacement liver (or thymus or other organ) doesn't necessarily have to look like or be structured in the same way as the original - it just has to do the same job as the original. This is perhaps more obvious in the development of wholly artificial electromechanical organs than for tissue engineering, but it's still the case there as well.
Here is some interesting research that illustrates this point. In some cases specialized cell populations within an organ's structure are the important component of that organ, and thus to replace the organ's functions it is sufficient for those cells to exist in some useful location:
Lymph nodes can provide a suitable home for a variety of cells and tissues from other organs, suggesting that a cell-based alternative to whole organ transplantation might one day be feasible. [Researchers] showed for the first time that liver cells, thymus tissue and insulin-producing pancreatic islet cells, in an animal model, can thrive in lymph nodes despite being displaced from their natural sites.
In the study, [researchers] tested the possibility of using lymph nodes, which are abundant throughout the body and have a rich blood supply, as a new home for cells from other organs in what is called an "ectopic" transplant. They injected healthy liver cells from a genetically-identical donor animal into lymph nodes of mice at various locations. The result was an enlarged, liver-like node that functioned akin to the liver; in fact, a single hepatized lymph node rescued mice that were in danger of dying from a lethal metabolic liver disease. Likewise, thymus tissue transplanted into the lymph node of mice that lacked the organ generated functional immune systems, and pancreatic islet cell transplants restored normal blood sugar control in diabetic animals.
"Our goal is not necessarily to replace the entire liver, for example, but to provide sufficient cell mass to stabilize liver function and sustain the patient's life. That could buy time until a donor organ can be transplanted. Perhaps, in some cases, ectopic cell transplantation in the lymph node might allow the diseased organ to recover."