It could be a main bearing as well, if it needs replacing it can spill out oil onto the clutch disk to make it slip, although this and the clutch slipping would be more noticeable in a higher gear, and if your mechanic was in there recently he should've noticed oil on the old disk. I'd work on bleeding the system first because it doesn't cost anything, if that doesn't work then take it back to your mechanic...that what you payed him for.