The former Norwich defender is available on a free transfer after rejecting The Blades' offer of a new deal.
Boss Neil Warnock wasted no time in finding a replacement for Ullathorne, with Shane Nicholson making the switch across the Pennines from Stockport.
Ullathorne has also attracted the interest of Swiss Cup winners Servette, French side Bastia and a number of English teams are also monitoring his situation.
The 29-year-old has already experienced playing abroad after spending six-months with Spanish side Osasuna and could fancy a return to the continent.
Meanwhile, The Blades have admitted defeat in their quest to sign Lahcen Abrami after their request for a work permit for the Moroccan striker was rejected.
