Summerbee, who first played for the club 10 years ago, played for the Robins against Nottingham Forest two weeks ago after joining on a two-week deal.
But the 33-year-old did not feature against Blackpool on Saturday, and King says the loan signing of Colin Heath from Manchester United will scupper a deal.
King said: "Nicky was at the game but he wasn't involved. I couldn't have him on the bench because Heath is here for three months.