Shumpert suffered the injury midway through the third quarter of the Knicks' 100-67 loss to the Heat on Saturday.
Shumpert was dribbling near midcourt and changed direction while attempting a behind-the-back move. His knee gave out and he clutched it immediately, falling to the ground.
Shumpert was on the floor for several minutes surrounded by his teammates. Eventually, the guard was carried off the floor by fellow Knicks rookies Josh Harrellson and Jerome Jordan. He went to a local hospital for the MRI. Surgery has not been scheduled but will take place in New York.
"It hurts your spirits when you see your best defensive player go down like that," J.R. Smith said. "We have a lot of people and hopefully someone can step up for him."
"My prayers are out for him," Amare Stoudemire said.
The loss of Shumpert is devastating for a Knicks team that is playing with Tyson Chandler at less than full health because of the flu. Chandler and Shumpert are two of the Knicks' most important pieces on defense.
Shumpert had emerged as the Knicks' top on-ball defender in his rookie season. He was assigned the difficult task of guarding Miami's Dwyane Wade in the Knicks' first-round series.
It's unclear if Smith or Landry Fields will replace Shumpert in the Knicks' starting lineup.
The injury also has long-term implications for New York. Shumpert is expected to be out until the beginning of the 2012-2013 season, at the earliest.
If the injury forces him to miss the first few weeks of the regular season, the Knicks may have to go outside of the organization to replace him at shooting guard.
The team could opt to go with Fields, but the second-year guard struggled for large stretches of the season. He will be a restricted free agent in the summer. There is a chance that the Knicks will let him walk.
Another option is Smith, who has a $2.5 million player option for next season. However, two league sources familiar with Smith's thinking say he likely will decline the option and test free agency.
So the Knicks may have to search for a replacement for Shumpert, who was selected with the team's first-round pick in the 2011 draft.
Shumpert led all rookies with 1.7 steals per game in the regular season and averaged 9.5 points per game.
Ian Begley is a regular contributor to ESPNNewYork.com.