The 1.88m striker has been playing with the Minnesota Thunder in the United States.
A former Christchurch Rangers player, Knox (29) studied at Macalester College in Minnesota before progressing into the professional ranks.
Knox was twice voted on to the Division 3 All-American team while at Macalester.
He then signed for the Thunder, the same club as former Otago players Tony Alvino and Kevin Taylor, and also played for the Seattle Sounders and the Atlanta Silverbacks, before returning to Minnesota.
A former Canterbury player, Knox played for Mainland in 2006 when present Otago player Stu Kelly was also turning out in red and black.
"Another Canterbury player," Fleming said with a smile, referring to the large group of Christchurch-based players in his Otago squad.
"He was keen to sign, and he is a class striker with physical presence.
"He will join our Christchurch contingent, train there and travel south on Fridays for home matches. I am pleased with the fitness levels being achieved up there."
There was no lack of stamina in last week's respectable draw with defending champion Auckland City.
Rarely has an Otago team exerted 90 minutes of stifling pressure on an opponent.
"The game plan was capital P for pressure, and the guys were in their face until that final whistle," Fleming said.
"We also could have caused a huge upset if one of our several goal-scoring chances had gone in."
Otago plays Hawkes Bay in Napier tomorrow, and while coach Matt Chandler's side may not have the glitter of Auckland City, the Bay has an intimidating record of seven wins over Otago in the past three years.
However, the pain of Hawkes Bay's 4-2 loss to Waikato last week was compounded by the loss of defender Bill Robertson with a leg fracture.
The hasty signing of English defender Stuart Ferguson, pending international transfer clearance, should shore up the Bay's defence.
Within a couple of weeks, Otago will have Andy Coburn back from suspension and Knox will be up to speed.
Christchurch Golden Boot winner Dan Ede has already recovered from injury, and could get a run against Hawkes Bay tomorrow.
Fleming is considering shaking up his starting XI to let him assess the potential of his bench, but said it was imperative United maintained the energy it showed last week.
Otago Utd v Hawkes Bay
Napier, tomorrow, 1pm
Otago United: James Waggett, Rhys Henderson, Tristan Prattley, Dave Shaw, Chris Jenkinson, Tom Sadd, Robbie Deeley, Stu Kelly, Ryan Faichnie, Dave Dugdale, James Reichwein, Brian Brown, Dan Ede, Tom Connor, Tim Horner.
Hawkes Bay: Shaun Peta, Sam Halligan, Phil Clarke, Chris Greatholder, Jonathon Taylor, Matt Hastings, Leon Birnie, Dan Wilson, Joachim Rande, Andrew Abba, Sam Jacob, Marama Thompson, Fergus Neil, Daniel Ball, Chris McIvor, Stuart Ferguson, Richard Gillespie.
