Sunderland boss Steve Bruce claims 'parasites' affected Asamoah Gyan's form after his goal in the 1-1 draw with England at Wembley in March.
Throughout the transfer window, the Ghanaian, 25, was constantly linked with a move to a big European club.
But the Black Cats say they did not receive a single bid, and Bruce has slammed agents for turning Gyan's head.
"Since that game at Wembley, all the parasites, as I call them, hover around," said Bruce.
“I had a conversation with him two days ago to say, 'Look, the window as now closed, Asa. We need to see you back playing again and back the way you know you can play. ”
"People are in his ear constantly trying to engineer a deal for him. Certainly since the England game,
when he played at Wembley so well on the night
, something has been troubling him.
"It's very difficult, the constant speculation no matter what you try to quash or quell, and the people around him, the people who want to make a fast buck, and it affects him in the end.
"He was going from Real Madrid to Bayern Munich to Valencia to Atletico Madrid."
Without any offer, Gyan will stay at Sunderland
where his contract runs out in 2014
Now Bruce has challenged the striker to rediscover the form that brought him 10 league goals following his move from French side Rennes in August 2010.
"We have had a discussion with him and he needs now to focus and get himself back on track again and be the Asamoah Gyan we know he is capable of," said the Sunderland boss.
"These things affect these players, and it has affected Asa.
"I had a conversation with him two days ago to say, 'Look, the window as now closed, Asa. Right, we need to see you back playing again and back the way you know you can play'."
Gyan - who has yet to score this season - will miss Saturday's Premier League match with Chelsea because of a hamstring problem picked up on international duty with Ghana.