Sportal.co.nz 27.Jun.2012Getty Images
With Horwill (hamstring) out for the rest of the season and the Reds needing to win their final three games to have a shot at making the finals, Wallace-Harrison has won the number five jersey ahead of Radike Samo and Van Humphries.
McKenzie said he had consistently been impressed by the work ethic of the 32-year-old Wallace-Harrison, particularly in the three-week international break prior to matches against the Rebels, Highlanders and Waratahs.
"(Wallace-Harrison) was our best player against the Hurricanes (who defeated the Reds 36-7 in an exhibition match last Friday)," McKenzie said.
"He is a very learned guy and he has put a lot of thought and effort into the last couple of weeks.
"He is the one doing all the homework for us and he has always been that way.
"He always has a great impact for us coming off the bench and he is the right guy to replace James (Horwill) in the circumstances."
While Wallace-Harrison has been holding the fort, Reds training sessions have been patchwork efforts over the past three weeks, with no less than 10 players on Wallabies and Australian under-20s representative duties.
Wallace-Harrison's dependability will see him handed the vice-captain role as the Reds chase their fourth Super Rugby win in succession against a much improved Rebels outfit likely to include Kurtley Beale, James O'Conner and the retiring Stirling Mortlock.
Coach McKenzie said he expected his representative stars, including playmaker Quade Cooper and flanker Liam Gill, to slot straight back into the Reds' structures.
"We have worked really hard during the break to make sure the guys coming back can just walk in and get it done," he said.
"The thinking side of the game (against the Rebels) is already done - we just need the boys to put on their Reds gear and get on with it."
McKenzie said the need to finish the season undefeated was something he had instilled in his men well before the international break.
"That challenge has been there for a while now," he said.
"I remember talking to the boys about going six-from-six and we have pushed through three of those and got to the back end of it.
"The Rebels are a tough ask down there but hopefully they have the brass bands out and get all engrossed in the emotion (of Mortlock's farewell) and we sneak in there and score 50 points."