Armed Racing Club fans stopped a player in the street and threatened to shoot him in the legs if their team failed to win their next game, a team mate who witnessed the incident said.
Giovani Moreno and Federico Santander had just pulled out of Racing Club's training ground in their car when another vehicle pulled in front of them and forced them to stop, Santander said.
Two men got out and threatened Moreno with a gun.
"They told Gio they would shoot him in the legs and would ruin his career," Paraguayan Santander told a radio interview late on Tuesday.
"I thought they were going to shoot him. He was terrified as I was. I'm frightened, I don't know what to do.
"They said that if we did not win the game, we would suffer the consequences."
Moreno's agent Agustin Jimenez told the Fox Sports cable channel: "They put a gun against his knee."
The incident happened after it was reported that Colombian Moreno could leave the club and join rivals River Plate.
Racing, who face Belgrano on Saturday, have dropped to 16th in the Clausura championship after a string of poor results.
They are 14th out of 20 in the relegation standings which are decided over three seasons.
Argentine clubs are followed by groups of hardcore fans known as barras bravas, who are blamed for the country's chronic hooliganism problem.
They are often given free entry to matches and their travel to away games is subsidised by the club.
Many groups are split into factions who sometimes fight amongst themselves and often side with club directors in internal political disputes.
They also threaten and attack players of their own team when results go badly.
In February, a junior player with relegation-threatened San Lorenzo was knifed in the stomach during an argument with a group of youths near the stadium.comments