Arbeloa came into the club as a replacement for the sacked Xabi Alonso but his first match didn't go exactly to plan.

A weakened Real found themselves a goal down to a 42nd minute opener from Javi Villar before Franco Mastantuono levelled the score with his goal in the third minute of injury time at the end of the first half.

The likes of Kylian Mbappe and Jude Bellingham weren't included in this tie as Arbeloa rested certain key players for their upcoming La Liga and Champions League clashes.

Jefte Betancor restored Albacete's lead on 82 minutes, and despite a 91st minute Gonzalo Garcia equaliser, a second goal on the night from Betancor, just three minutes later, sealed the shock win.

Reflecting on the loss, having only taken charge of the team on Monday, Arbeloa said on IOL: "Here at this club, a draw is already bad – it's a tragedy. Imagine a defeat like this, it's painful.

"I'm sure all our fans feel the same way. Even more so when it happens against a team from a lower division, although we already know here how tough any opponent can be.

"If anyone is responsible and to blame for this outcome, it's clearly me, the one who made the decisions regarding the lineup, how we wanted to play, the substitutions.

"I can only thank the players for the way they welcomed me, for the effort they put in today."

Elsewhere, Real Betis beat Elche 2-1, with a brace from Ezequiel Avila (68', 80') to cancel out Leo Petrot's 58th minute opener.

And Alaves overcame 10-man Rayo Vallecano 2-0 to secure their place in the quarterfinals.

Toni Martinez was on target for Alaves five minutes into the second half before Isi Palazon was dismissed with 69 minutes played. Carlos Vicente then sealed the result with an 89th minute strike.

Round of 16 results

Real Sociedad 2(4) - 2(3) Osasuna

Cultural Leonesa 3-4 Athletic Club

Deportivo 0-1 Atletico Madrid

Real Betis 2-1 Elche

Alaves 2-0 Rayo Vallecano

Albacete 3-2 Real Madrid

Fixtures

Burgos v Valencia

Racing v Barcelona

FECAFOOT President Samuel Eto'o sanctioned by CAF for misconduct