None of the big men available are better then the Celtics under contract big men. (Not counting two way contracts because neither of the ones talked about could be signed to one of those)
If for some reason one of those three big men were moved in a trade that opened up a slot, then yes, it is time to think about adding one of them for the depth.
But Len was cut from the Raptors who are struggling at the C position. Dedmon has been available all season and no one has sign him. There are usually reasons.