Former University of Minnesota quarterback Philip Nelson was arrested early Sunday at the scene of a downtown Mankato assault that left another well-known local football player critically injured.

Nelson, 20, was arrested on probable-cause assault about 2 a.m. after Mankato police patrolling the downtown entertainment district found Isaac ­Kolstad, 24, a former player at Minnesota State University, Mankato, unconscious after being kicked twice in the head.

Police are looking for a second man who they believe was involved but offered no details on what roles Nelson and the other man played. A recording of conversations between police and dispatchers indicates that three people at the scene said Nelson was one of the assailants.

On Sunday evening, Kolstad remained in critical condition at Mayo Clinic Health System-Mankato. A former linebacker, he graduated in December with a major in business management and administration and works as a sales representative for ­Fastenal.

Kolstad’s younger sister Rebekah tweeted, “Please everyone who reads this pray for my big brother Isaac he is in ­critical condition and needs your prayers to help him survive.”

The Kolstad family later issued a statement through Mayo, requesting privacy for his wife, children and family.

“We are extremely grateful for the outpouring of support we have received during this difficult time,” it said. “Isaac is a very strong and determined young man, who is dearly loved by his family and friends. We would respectfully ask for your continued prayers.”

A CaringBridge site set up included a note from Kolstad’s father, Blaine, saying that his son was fighting for his life.

“He is young and strong, but the battle he has in front of him is enormous,” the father wrote. “He had one surgery today to put a pressure gauge in his skull. He may need additional surgeries in the coming days. 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.”

MSU, Mankato football coach Todd Hoffner called Kolstad “a tremendous young man.”

“He’s a competitor and a great quality person,” Hoffner said. “All of our thoughts and prayers, and all of our hopes are for a full recovery for him.”

As rain pounded outside, the team held an indoor candlelight vigil for Kolstad Sunday night. About 200 people attended, holding purple signs saying “#22STRONG.” Kolstad wore No. 22 as a Maverick.

Speakers urged people to pray for Kolstad. “He’s in critical, critical condition, to put it bluntly,” one friend said.

Nelson’s family members could not be reached for comment. Nelson, who in January transferred to Rutgers University, was a high-profile recruit who started 16 games for the Gophers in 2012 and 2013. He participated in spring football practice at Rutgers, which is joining the Big Ten this fall, but he would have to sit out a year before he could play.

The incident apparently followed a busy night downtown when people were out celebrating graduations. Jonny Christensen, a former MSU, Mankato football player who tends bar at the South Street Saloon, said that he talked with Kolstad and a couple of other “football guys” shortly before the bar closed.

Christensen said he had no idea what happened after that. The saloon is about half a block from where police found ­Kolstad. “I talked to him an hour before the incident ­happened,” he said.

“He wasn’t drunk … He was happy and he was dressed up, but he’s not a regular downtown. He’s a … quiet young man, with a new wife and daughter.”

Christensen has stayed in touch with the football program and described Kolstad as “not the type to get all crazy. He’s so low-key and calm. I’ve never seen the guy without a smile. I’m just crushed. A lot of people are.”

The assault occurred near the intersection of Cherry and South Front Streets, in an area where there are many security cameras, so the incident may have been caught on tape.

Nelson, Minnesota’s Mr. Football in 2011, played at Mankato West High School. Kolstad, four years older, was at Mankato East. It is unclear whether they knew each other.

Through a team spokesman, the Gophers declined to comment. Rutgers issued a statement saying, “We are in the process of gathering information on the situation and reserve comment until the legal process is complete.”

It is unclear when Nelson will be arraigned. Police will make a recommendation to the Blue Earth County attorney’s office.