As a promising modulation scheme for high mobility wireless communications, orthogonal time frequency space (OTFS) has received tremendous attention. In the case of bi-orthogonal waveforms, OTFS allows highly efficient detection in the delay-Doppler (DD) domain by exploiting the (circulant or sparsity) structures of the channel matrix. However, in the case of more practical waveforms, such as the rectangular waveform, the special structures do not exist. In the literature, to achieve low complexity detection, null symbols or cyclic prefixes (CPs) are frequently inserted to the OTFS signal block, which, however, leads to significant overhead. In this work, we partition the large channel matrix in the time domain into a number of small sub-matrices (blocks), and represent the system with a factor graph. Based on the graph representation, a message passing receiver is developed, where the unitary approximate message passing (UAMP) is used to handle the blocks. The proposed receiver does not require the frequent use of CPs or null symbols, while delivering promising performance.