The Venezuela international has struggled to make an impact since arriving on loan from Getafe last August.
Miku has scored twice in 14 appearances for Celtic, and his chances have been limited due to the competition for places.
Georgios Samaras, Tony Watt and Lassad Nouioui have all partnered Gary Hooper at various points throughout the campaign, and the return of Anthony Stokes from injury has further curtailed Miku's chances of a run in the team.
Miku said: "It was a mistake to go there. Simply because I cannot do anything.
"At first I did not understand. I said to myself: 'How might come to play in the best league in the world (Spain) and play no part in Scotland?'
"But then I realised that things had nothing to do with sport."
And the 27-year-old criticised the Celtic coaching staff in a newspaper interview published in his home country.
Miku added: "We do not talk. I like people to tell the truth. I don't like that they say one thing to my face and another thing behind my back.
"I know you are going to play those who are there, who know the coach and they have more confidence. If there is an injury, I know I can participate."