McAllister, 24 joined the Bramall Lane club twelve months ago after signing from Irish club St. Patrick's Athletic and to date has featured in ten first team matches. He has joined League Two outfit Shrewsbury Town and could hake his Shrews debut tomorrow when they entertain Gillingham.
Meanwhile Argentine defender Elian Parrino has left the club by mutual consent after failing to make any first team appearances under Blades boss Danny Wilson.
Former United defender Robert Kozluk has joined Bradford City after a successful trial; he had been training with Rotherham United but opted to join the Valley Parade club after being released by Port Vale last week.