Are you certain they knew he wouldn't be back earlier? Or is it possible that injury timelines aren't consistent from person to person and that new information becomes available?
I think fans are giving way, way, way too much credit in thinking doctors are unbelievably good at prognosticating future scenarios.
Putting him on IR does what, exactly? Who's this high quality player who's not playing because Lamar is inactive each week? And if Pittsburgh is the earlier he comes back, that's why you don't put him on IR. IR means Cincy is the earliest he can come back. IR also prevent him from practicing.