Banks received a surface wound to his side that required stitches and "remains in a local hospital for precautionary reasons and is expected to be released within 24 hours," his agent James Gould said in a statement Monday.
Banks had been expected to leave the hospital on Sunday.
Banks' friend, identifed as Christopher Nixon, was critically injured and underwent surgery, Gould said.
"Brandon's friend ... is recovering," Gould said Sunday. "He was talking today, and also is expected to make a full recovery."
The stabbing occurred shortly after 3 a.m. outside The Park at 14th, an upscale nightclub. According to D.C. police, a man got into a verbal altercation with two men, then pulled a knife and stabbed them. Police did not identify the victims, but the Redskins confirmed that Banks was injured in the attack.
The suspect -- Jason Shorter, 24, of Lanham, Md. -- was arrested and charged with assault with intent to kill, police said. Police and jail officials did not know whether Shorter had an attorney.
According to Banks' agent, the friend was attacked first and Banks was injured while coming to his aid. Banks was not implicated in any wrongdoing, Gould said.
"Brandon's immediate concern is for his lifelong friend who is like a brother to him," Gould said. "He would like to thank everyone for their prayers and well wishes for him and his lifelong friend."
The team was gathering facts about the incident, Redskins spokesman Tony Wyllie said.
Banks was an undrafted rookie out of Kansas State last season. He returned a punt 53 yards the first time he touched the ball in an NFL game. He also recorded a 96-yard kickoff return for a touchdown. He is the lightest player in the NFL -- listed at 5-7, 155 pounds.
Information from The Associated Press was used in this report.