The Northern Ireland international only could play 45 minutes of the game against Bury yesterday (Tuesday) and had to be substituted due to ankle problems. It has turned out that Keith has picked up ankle ligament damage which likely will see him miss the start of the season.
The Blades now don't have any real natural right wingers. Kyle Naughton and Greg Halford could be candidates but will it work out. Blackwell must be persuaded to now pay over the odds for David Cotterill.
Blackwell said: "We do have strength in depth with Greg Halford back in contention this week and Kyle Naughton doing well."
Gary Naysmith also had to leave the game against the Shakers prematurely because he was hobbling around the pitch. His injury has not yet been stated.