Blaine Kolstad, Isaac's father, posted on caringbridge.org Sunday night that his 24-year-old son is in critical condition, has had surgery to put a pressure gauge in his skull and may need additional procedures.
"He is currently in critical condition and fighting for his life," Blaine Kolstad wrote. "He is young and strong, but the battle he has in front of him is enormous. ... We do know that his brain did sustain permanent damage. We do not yet know to what extent and won't for many days. He is very sick. Please pray for Isaac and all of those involved."
Nelson, who transferred to Rutgers from Minnesota earlier this year, was arrested at the scene and jailed early Sunday on suspicion of assault after officers found Kolstad in the Mankato downtown entertainment district at 2 a.m.
Steph Stassen, a graduate student who said she witnessed the assault, told the Minneapolis Star Tribune that the bars had just closed when "all of a sudden, this guy comes out of nowhere and sucker-punches" Kolstad. Stassen and her roommate said another person, not Nelson, threw the punch, but that a man she later learned was Nelson then kicked Kolstad in the head "like it was a soccer ball -- just out of nowhere."
Kolstad was taken to Mayo Clinic Health System in Mankato. He graduated from Mankato East and spent two seasons at North Dakota State in 2008 and 2009, did not play in 2010 and transferred to Minnesota State for his junior and senior years. He started all 12 games last season and finished third on the team in tackles with 59.
A candlelight vigil for Kolstad was held at Minnesota State on Sunday evening.
Sam Thompson, who said he played football with Kolstad for three years, told The Associated Press he was with Kolstad on the night of the assault but didn't want to comment out of respect for his family and the investigation.
He did say Kolstad has a 3-year-old daughter and that his wife is pregnant.
"A recurring theme in his life is the importance of family," Thompson said. "He's got such a beautiful family that he's just starting. ... He's just a family guy. He loves his daughter so much and is looking forward to the birth of his second child."
Police are requesting felony charges of third-degree assault against Nelson, KTOE-AM reported.
Nelson, 20, played two years at Minnesota and started nine games last season before announcing in January that he would transfer. He participated in practice at Rutgers this spring and is set to sit out the 2014 season because of NCAA transfer rules.
He played in 11 games last season but said Rutgers offered a more pass-friendly offense. He completed 50.5 percent of his passes for 1,306 yards with nine touchdowns and six interceptions last season. He also rushed for 364 yards and six touchdowns while sharing quarterback duties with Mitch Leidner.
Rutgers said in a statement that it is in the process of gathering information about the incident. Nelson, who was Minnesota's Mr. Football at Mankato West High School, likely will be arraigned Monday or Tuesday.
Police are looking for a second suspect.
"It's an ongoing investigation, and as more information becomes available, we'll be releasing it," city spokeswoman Shelly Schulz said Sunday.
Information from ESPN.com's Adam Rittenberg and The Associated Press is included in this report.