The Manchester City defender was recalled to Gareth Southgate's squad after more than a year in the cold but was shown a second half red after two bookings, the second for a reckless sliding challenge.
Having been left out of recent squads, and with competition from Kieran Trippier and Trent Alexander-Arnold, Walker's career looked to be over.
The 30-year-old could have won his 50th Three Lions cap against Denmark on Tuesday but will now miss the game through suspension and raised concerns that his tally could remain at 49 for good.
Speaking to Sky Sports an emotional Walker said: "To get sent off for your country is never a nice moment, especially when you're on the caps that I am, 49, and being out of the team for so long. I should know I am on a yellow card and to make a rash challenge like that, it's not acceptable.
"That's the sad thing about being a footballer, it does come to the end some time and I am probably near the end of my international career.
"I am still very energetic, still feel like I've got a lot in the legs and hopefully I will be back."
Walker will discuss his role in the camp with Southgate following the game but insisted he was more than happy to remain and train despite being unable to feature on Tuesday in Copenhagen.