Complex sentence: This fact has brought a lot of question to the exact role of van Eyck's signature; if it is taken that the work is unfinished, then the completion refers only to the design of the picture, left to be finished by members of his workshop.
Split into simpler sentences: This fact has brought a lot of question to the exact role of van Eyck's signature. If it is assumed that the work is unfinished, then the completion may refers only to the design of the picture, left to be finished by members of his workshop.