King completed full training at Moorabbin on Friday, proving his fitness for Sunday's crucial match at Marvel Stadium.
He was named in the extended squad, alongside Brad Hill, but the Saints wanted to be sure the 23-year-old was ready to go.
King needed a reconstruction on his right shoulder in the pre-season and returned in round 10, playing seven games.
He dislocated the same shoulder early last month in the loss to Melbourne, with St Kilda initially fearing the injury spelled the end of his season.
Hill was declared a certain starter on Thursday night after missing last week's win over Hawthorn following a heavy knock against North Melbourne in round 19.
After starting as the sub against the Hawks, free agent Jade Gresham has been promoted to the 22, with Liam Stocker and Jack Billings omitted.
Carlton's injury woes have continued as they chase a seventh-straight win, with defender Mitch McGovern joining midfielder Adam Cerra sidelined with a hamstring issue.
Enigmatic forward Zac Fisher has received a reprieve after playing just eight games for the Blues this season.
19°C