Rodriguez, 28, will be charged with third-degree assault, Lt. Mike Wysokowski said.
"He was arrested at 10:15 after a domestic dispute in the family area with what appeared to be his father-in-law," Wysokowski said. "The victim suffered facial abrasions and was taken to Flushing Hospital."
Rodriguez was being held at Citi Field, Wysokowski said.
"There was an incident at the ballpark between Francisco Rodriguez and his family," Mets vice president Jay Horwitz said in the empty team locker room about 90 minutes after the Mets lost to the Rockies. "He was questioned by police. Other questions pertinent to this matter we refer to the police department."
Rodriguez did not pitch in the game, which the Mets lost, 6-2. Jerry Manuel was questioned after the game about why he did not use Rodriguez in the eighth inning, when reliever Manny Acosta surrendered a grand slam to Melvin Mora.
An irate Rodriguez refused several interview requests after the game, at one point telling a reporter to "get out of here" and at another point telling a group of reporters, "I'm not talking. Did I -- pitch? Then why do I have to talk to you guys?"
Rodriguez then left the clubhouse and went into a private area where reporters are not allowed. Moments later, police officers entered the clubhouse and went back to the area.
The Daily News reported on its website that Rodriguez earlier was involved in a heated altercation in the hallway outside the family lounge, where family members wait for players.
Moments after that, Emergency Services personnel, police and Citi Field security tended to Rodriguez's father-in-law in the stadium's interview room, which is adjacent to the family lounge. A Rodriguez family member was taken out of the stadium on a stretcher and loaded into an ambulance.
Third-degree assault is a Class A misdemeanor punishable by up to a year in jail. He will be arraigned in Queens criminal court today.
The chaos followed one of the more frustrating defeats of the season for the fading Mets. Once again, they failed to win back-to-back games. The Mets have not won two in a row since June 22-23 against the Tigers.
It's a maddening streak that looked as if it would come to an end when the Mets held a 2-1 lead with two outs and no one on in the eighth.
Hisanori Takahashi (7-6), in his second day as the Mets' latest eighth-inning guy, allowed a single to Todd Helton and walked Carlos Gonzalez. Manuel called on Acosta to face Troy Tulowitzki. Rodriguez, who threw 18 pitches Tuesday, was not asked to attempt a four-out save.
"Usually when we have K-Rod the night before, we don't expect him to have four or five outs the next night," Manuel said. "We try to limit it to one inning. If it had been a situation where he had not pitched in a couple days . . . we would take that shot."
Acosta threw a wild pitch before Manuel ordered an intentional walk to load the bases for the righthanded hitter Mora. Jim Tracy, with lefthanded hitters Brad Hawpe and Jason Giambi available, let Mora bat. Good move. The former Met rocketed a 2-and-2 pitch over the leftfield wall for a grand slam and a 5-2 Rockies lead.
"We've got to find a way to get that out," Manuel said. "With the way we are offensively, it allows us no mistakes . . . We have no margin for error.''
The Mets had three hits, none after the fifth inning. Eight of their last 11 outs were strikeouts.
With Ken Davidoff and Mark Herrmann