Olympiacos will face United without former top scorer Konstantinos Mitroglou, who signed for Fulham in January. While that sale has obvious benefits, Moyes concedes it also poses a scouting dilemma.
"They are really good and have had a great record. They are undoubtedly the best team in Greece, no question about it. They also have a lot of Greek internationals who will go to the World Cup and it will be a really tough game," he said.
"Mitroglou was a really good goalscorer for them and a main player. Does it weaken them? I think they'd rather have him than not but probably felt the offer was so good they had to take it. Since then, they've brought in two or three other players as well and I couldn't be sure who will and won't play."
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