Karanka's autocratic management style made enemies and Gibson has charged Agnew with the task of restoring harmony and saving Boro's Premier League skin. But will it keep them up?
Boro have spiraled down the table of late and find themselves 19th having last won a league game on December 17.
However, his team have struggled in the Premier League this season, now sitting 19th with just four wins from 27 games.
Karanka was hired in 2013, which marked his first job in management following a spell as Jose Mourinho's assistant at Real Madrid.
In a statement on their official website, Middlesbrough revealed that both they and Karanka agreed it was in the "best interests" of the club to part company.
Aside from Agnew, former Boro defender Nigel Pearson, former England managers Steve McClaren and Roy Hodgson, former United States manager Jurgen Klinsmann and former Crystal Palace boss Alan Pardew are among the leading candidates to replace Karanka on a permanent basis, according to bookmakers.
With only 19 goals from 27 matches, and just four wins all season, 'Boro have scored the fewest goals in the top flight despite keeping a tight defence. "We wish him all the best for the future".
"This club will always hold a special place for me and I wish everyone connected with Middlesbrough Football Club the very best for the future".
But it seems that Manchester United supporters are more devastated than anyone at Thursday morning's news - more so even than resigned Middlesbrough fans or Karanka himself.