Browns coach Romeo Crennel said Winslow, who has been plagued by injuries throughout his NFL career, has been sick for two days. Crennel would not say if Winslow would play in Monday night's game against the New York Giants.
"He's a valuable piece to the puzzle we have," Crennel said Thursday. "I know that he'll want to play, so he'll do everything he can to be able to play."
Crennel would not elaborate on Winslow's condition or disclose any medical problems.
"Not until after doctors take look at him and see if they can know what it is," he said.
Team spokesman Bill Bonsiewicz said the team would have no further update on Winslow until Friday, when Crennel holds his news conference.
Winslow's agent, Drew Rosenhaus, did not immediately return a message seeking comment.
Winslow caught 82 passes and made the Pro Bowl last season, his fourth as a pro. So far this year, he has 19 catches and one touchdown for the Browns (1-3).
Winslow was kept out of practice last week when the Browns had a bye. He practiced on Monday and was at the team's training facility on Wednesday but did not participate in the club's workout. This past offseason, Winslow had arthroscopic surgery on his right knee.
He has had other procedures on the knee, including one to clean out a staph infection he got while recovering from injuries sustained in a near-fatal motorcycle accident that caused him to miss the entire 2005 season.
Meanwhile, wide receiver Donte' Stallworth practiced and seemed confident he would make his season debut against the unbeaten Giants.
"It's coming along, getting better," Stallworth said. "I feel I can go out and be effective for the team and not be a liability."