Albion manager Graham Potter has refrained from including Connolly, 21, in his matchday plans on a regular basis this season, with the player sure to welcome the opportunity for regular minutes on the pitch.
Connolly has only featured four times in the 21/22 Premier League campaign - starting only once - and has not been included in the previous two matchday squads.
Brighton's issues in front of goal have been evident despite impressing this season, and it is expected that offensive options will be pondered in the January transfer window.
Despite the need for goals, Potter has decided that Connolly's development is better suited somewhere else, where the chance to lead a revitalised frontline in the second division can kickstart his career.
A fitting move for all parties
Middlesbrough manager Chris Wilder will no doubt welcome the addition as he seeks to bolster his firepower in the club's pursuit of a return to the top division.
The Boro currently sit in fifth place in the Championship with four victories in their past five games, include a thrilling late steal over promoted Blackpool in their last fixture.
Despite a noticeable upturn in form since Wilder's arrival, replacing Neil Warnock in November, there remains a need for offensive additions to aid the club through the latter period of the league term.
Of the seven teams at the forefront of the table, Middlesbrough hold the lowest tally for goals scored with 29 in 25 matches, and Connolly could prove key to climbing this particular tally.
Since then, opportunities and performances have seen a decrease in inclusions and the chance to reignite a promising career will be welcomed by all parties involved.