Meanwhile for Jose Mourinho's visitors, summer signing Henrikh Mkhitaryan has become an integral player under the "Special One" after initially struggling to adapt in English football following his move from Borussia Dortmund.
Mourinho hit out at Premier League bosses for their scheduling of the Boro match so soon after the 1-0 win over the Russian side, with Paul Pogba, Daley Blind and Marcos Rojo all picking up injuries during that match.
Middlesbrough chairman Steve Gibson has denied Karanka was sacked, instead saying the 43-year-old Spaniard "sacrificed himself" for the club in a mutual decision to step down.
Head coach Karanka left Boro, who sit in 19th place in the Premier League table, on Thursday.
Asked how to get the best out of Downing at a pre-Manchester United press conference, Agnew, who believes he is capable of keeping Middlesbrough afloat, told reporters: "Stewart's a Middlesbrough boy. I think on the fourth year he deserved to be sacked".
Mourinho would not name names, but it is clear that he feels that the Middlesbrough players pushed Karanka out of the club.
"In the first year when he arrived, the team was going to League One and he saved the team". In the second year, he took them to Wembley to play in the play-off final.
Assistant Steve Agnew has been placed in charge ahead of Sunday's clash at home to Manchester United.
"That may disappoint those people who want glamour and somebody to come in ready-made, a sexy name, but that's not what we need at this point - he's more than a safe pair of hands", said Gibson.
Meanwhile, Boro have announced that Joe Jordan has been appointed to help caretaker boss Steve Agnew.
The former Republic of Ireland worldwide continued: "There are names [being mentioned] galore, but I would personally go for Martin O'Neill, that would be my gut instinct".