As we previously reported, the superstar LB was allegedly involved in an altercation with a female at the Niners' hotel Saturday night ... in which a woman says Reuben took her phone and slapped her in the face.

When the woman spoke with police, she told them Foster had also pushed her in the chest and hit her with an open hand on the left side of her face. Cops say they noticed a scratch on her collarbone.

The 49ers cut Foster following the arrest ... with coach Kyle Shanahan saying, “Our number one rule is you got to protect the team and he’s put us in a bad light too much.”

Redskins Senior VP of Player Personnel Doug Williams said in a statement they're very aware of the severity of the accusations, but say Foster could be worth the risk regardless.

"We decided to investigate the situation with Reuben further by claiming his rights after candid conversations with a number of his ex-Alabama teammates and current Redskins players who were overwhelmingly supportive of us taking this chance."

Doug added, "Let me be clear, Reuben will have to go through numerous steps including the full legal process, an investigation and potential discipline from the NFL, as well as meetings with counselors associated with the team before he will ever have the opportunity to wear the Burgundy and Gold as a player."

"Nothing is promised to Reuben, but we are hopeful being around so many of his former teammates and friends will eventually provide him with the best possible environment to succeed both personally and professionally."