Warren Gatland, the new Lions head coach, will announce his squad to tour Australia on April 30 and McGeechan is confident the New Zealander, who he had as his assistant coach in 2009, is the right man to make the tough calls.
He told Press Association Sport: "You need to take fit players. With only a six-week preparation period you can't have players carrying injuries or players out of form."
He added: "That's the big call for the coaches, picking fit players who are on form who can then come together, so you are working with what you want to do right from the word go with players doing every training session and available for games.
"You can't find form and if you are injured you haven't got the time.
"That's been the mistake in the past, in that players who have not been in form or have been injured have gone because the coach thinks he's the best player in the position based on reputation, not performance.
"I think the three months prior to the tour is the critical period."
Wales coach Rob Howley will also be on the Lions coaching staff for a second tour, along with Andy Farrell and Graham Rowntree, and McGeechan says continuity is an important factor.
He said: "When you are trying to put together a top-class, world-class Test team, if you have got that little bit of continuity you are away from first base straight away.
"I think Warren Gatland was the right appointment, he's an outstanding coach but he also understands the Lions and has experience from 2009. So it will be lot of similar management people and maybe a third to a half of the players. You are not starting from scratch knowing you have five weeks to be Test match ready."