Ravel Morrison played for West Ham United for three years, but left to join Lazio over the summer.
Sullivan hosted an impromptu question and answer session on social media site Twitter earlier today, and he was asked by a Hammers supporter to give his opinion on Morrison.
The attacking midfielder is a player who divides opinion, but Sullivan had nothing but praise for the 22-year-old.
Morison played for West Ham for three years between 2012 and 2015, and he showed glimpses of his talent during that time.
The England youth international was magnificent in bursts during his time at Upton Park, but he was unable to sustain that form and was often shipped out on loan as a result.
In the end, Morison's attitude was brought into question and he was eventually allowed to leave in the summer of 2015, once his contract expired.
Morison is now playing for Lazio, but his time in Serie A has not been particularly productive either, making just two appearances for his Rome-based employers to date.