It's all on Lamb, not his manager. The manager was doing his job (although highly ambitious...) by chasing top dollar early in the season after a strong finish to 2017, but Lamb was unable to back it up on the field in 2018 when it mattered.
Newcastle were never going to pay Lamb the sort of money he or his manager thought he was worth, and if any rival club thought he was worth the dollars they would have signed him long ago. Unfortunately for Lamb, what everyone already knew in regards to his true value has only just dawned on him. It's not really a case like Blake Austin where he had an offer but held out for more, only to have it all slip away.
It's one of the reasons I'm happy with Brown playing Cogger ahead of Lamb. He backed his ability by taking a small contract at Canterbury where he has a clearer pathway to first grade, and he is obviously still putting in at training and earning his position in the team.