It appears that Russell Simmons isn’t the only one mad at Nike these days. Now they have NFLPA president Gene Upshaw riled as well.

A published report last week revealed that the NFL Players Association is a bit irked by letters that Nike sent to its NFL athletes under endorsement contract warning them to not wear Reebok hats on the sidelines. Reebok has the exclusive NFL deal for authentic and sideline apparel and headwear. The league moved to have baseball caps worn on the sidelines designated as part of the player’s uniform.
Nike also reportedly also urged the players to wear Nike under their uniforms. Reebok launched its NFL-branded performance undergear product this year.

Reebok CEO Paul Fireman, in the report published in the Portland Business Journal, also responded to Nike’s moves. “Nike is just trying to cause trouble in this one particular issue. It's sour grapes that they didn't get the [NFL apparel] contract.”

The article said that Nike has threatened players in the past with breach of contract for wearing Reebok hats on the sidelines, but the NFLPA tried to resolve the issue by making the caps part of the official uniform. It said Nike sent the letters to players again this year, warning them that wearing Reebok caps on the sidelines, at press conferences or during interviews “would be a violation of your Nike football contract and could result in the termination of this contract.”

Upshaw shot back with a letter of his own to players, saying, “It is inappropriate and unacceptable for Nike to threaten any player for wearing any part of his official game uniform on the sidelines”, while stating that the NFLPA is prepared to defend the player’s ability to wear the prescribed sideline gear.

Nike reportedly has shoe and apparel deals with at least half of the league's 1,800 players.

Late last month, the New York Daily News reported that Russell Simmons, founder of Phat Farm, accused Nike of attempting to raid his Phat Farm Footwear Collection searching for talent. Right…