Legg wrenched his right ankle when going for the tryline near the end of last Thursday night's game against North Harbour and was forced off the field.
He has since had the injury checked and is expected to be unavailable for two to four weeks.
Kenny has come into the squad and Otago coach Steve Martin said the Lawrence player should fit easily into the squad as he had trained with the team earlier in the year.
Otago takes on Manawatu in Palmerston North in its second straight Thursday night game.
Second five-eighth Luke Herden, who made his starting debut for Otago last Thursday, is also out of the game in Palmerston North.
Herden injured his upper left thigh and was forced to leave the game about five minutes before halftime, as he could not run properly.
Legg was injured in a tackle about 5m from the line after a good run from flanker Adam Thomson.
Touchline officials did not let Sean Romans back on the field to replace Legg, and Glenn Dickson went into first five-eighth and Chris Noakes was forced to move into halfback for the last five minutes.
The decision not to let Romans back on the field incensed Martin, who said the officials did not know the rules.
"It clearly states in the rules that substituted players can go back on for a permanently injured player.
"They can't in Super 14, but they can in this competition.
"I was adamant but the officials blocked us," Martin said.
"They admitted afterwards their mistake and luckily it didn't cost us any points."
Martin said the results over the weekend showed the closeness of the competition, and the fact any team could beat anyone else on their day.
"There are no easy games.
"It is going to be a very close competition.
"Teams are going to have to be up for every game."
He said Manawatu would have been buoyed by its upset win over Southland in Invercargill, and Otago would have to be on its game on Thursday night.
Manawatu scored a 25-23 win over the Southland side, to notch its second win of the season, and sits in sixth position on the competition ladder, just two spots ahead of Otago.
Manawatu has injury problems and the game against Southland ended in golden oldies scrums as both of Manawatu's starting props went off injured.
Both teams are due to be named today.