However, United are denying any interest in West Bromwich Albion midfielder Sean Gregan.
Sullivan is the likely solution to the goalkeeping problem with Kevin Blackwell keen to bring in an experienced pro alongside the promising Scott Carson.
"At the moment Gregan is not necessarily at the top of my list,'' said Blackwell, "I'm looking at all sorts of players and if the names of any quality player come my way I'll always be interested.
"Sean Gregan is someone I've been made aware of, but I've not really pursued that one at all. Sully has been training with us this week and has done okay, I'm hoping to keep him here, but now it's down to negotiations between the two clubs.
"I reckon I'll know one way or another within the next three to four days.
"I've also been linked with (Aliou) Cisse at Birmingham and some others I haven't even heard of,'' Blackwell added, "I did intend to go and have a look at Cisse for Birmingham against Sheffield Wednesday earlier this week, but he didn't play, while there's another lad with Premiership experience who could be an option.''
Sullivan is the likely solution to the goalkeeping problem with Kevin Blackwell keen to bring in an experienced pro alongside the promising Scott Carson.
"At the moment Gregan is not necessarily at the top of my list,'' said Blackwell, "I'm looking at all sorts of players and if the names of any quality player come my way I'll always be interested.
"Sean Gregan is someone I've been made aware of, but I've not really pursued that one at all. Sully has been training with us this week and has done okay, I'm hoping to keep him here, but now it's down to negotiations between the two clubs.
"I reckon I'll know one way or another within the next three to four days.
"I've also been linked with (Aliou) Cisse at Birmingham and some others I haven't even heard of,'' Blackwell added, "I did intend to go and have a look at Cisse for Birmingham against Sheffield Wednesday earlier this week, but he didn't play, while there's another lad with Premiership experience who could be an option.''