McKenzie, an ever-present in the Scottish side since the start of last year's tournament, was in severe pain with a damaged disc and withdrew on doctor's advice. Graham Ellis, of Currie, wins a first cap as his replacement.
The Scots could ill afford a weakening of their already suspect front row; while Dave Hilton, the loose-head prop from Bath, is now a seasoned front-row campaigner, Matti Stewart, the Northampton tight-head, is still an international novice. No one would describe McKenzie as an imposing physical specimen - at 5ft 6in and less than 15 stones, he is the smallest Test hooker in world rugby - but equally, no one could deny that he punches his weight at the top level.
News of McKenzie's injury reached the Welsh camp as they flew into Edinburgh last night. Jon Humphreys, the Welsh captain, was diplomatic when questioned about the absence of his direct opponent - "I understand Ellis is a very good player," he said - but he added: "I came up against McKenzie last year and found him an extremely tough competitor. In international rugby, the individual contests tend to be pretty even but I remember the 1996 game against the Scots in Cardiff as being very hard in the front row."
At least the Scots were optimistic of Townsend's chances of turning out. Duncan Hodge, the uncapped goal kicker from Watsonians, was still with the squad last night after his precautionary promotion from the A team, but the feeling at Murrayfield was that the back division would appear tomorrow as selected.
Nine tenths of the Scottish rugby public appear to believe that Townsend should play at outside-half rather than outside-centre, but Kevin Bowring, the Welsh coach, was wary of questioning the logic of the home selectors in putting so much space between their most exciting attacking weapon and the forwards at the heart of the action.
"You ask me if it's a relief that Gregor will not be wearing the stand- off's shirt, but I'm not so sure it's good news for us," he said. "He's a threat wherever he plays on the pitch and we are well aware of the damage he can cause out wide.
"What I do know is that the Scottish coaches have worked very hard on Craig Chalmers, a very experienced outside-half in his own right, and they believe he can move the ball wide at the right times. It's a formidable proposition with both Chalmers and Townsend involved."
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