OM thumped Villarreal 4-0 i in the first-leg of their Europa League round 16 tie on Thursday night.
Marcelino said: "We have been far from our best version, the opponent has been good, we must congratulate them, it is a tough, very tough defeat. We expected a different result, I am the person most responsible for this defeat."
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"Decisions can only be made once and today I haven't made the best decisions.
"We made a lot of mistakes. Until the day in Las Palmas we had had a lot of problems. Since then we had been very effective, conceding little to the rival and we made big mistakes against the worst rival.
"We are going to think that there is a second leg left and that there are things to improve."