The Tigers accepted Southampton’s £12m offer for the Republic of Ireland forward during the week, making a quick profit on the man they signed in January.
But with European football now an issue at the KC Stadium, Bruce is desperate to get in a new striker to partner Nikica Jelavic.
An £8m offer for Fletcher was lodged over the winter, but Hull were unable to land him as they instead took Long from West Brom.
The Scot spent large chunks of last season out injured, but his proven Premier League goalscoring record with Burnley, Wolves and the Black Cats makes him a tempting option for the Humbersiders.
Sunderland themselves may be keen to cash in on their star, with Gus Poyet desperate to land £14m-rated Liverpool attacker Fabio Borini.
The 27-year-old himself could also be tempted by Europa League football with last season’s FA Cup finalists.