Wales boss Gatland is set to succeed McGeechan as the tourists' head coach once the details of his temporary release from the Welsh Rugby Union have been finalised.
Outstanding Wales flanker Sam Warburton has been installed as early favourite for the post given his close relationship with Gatland.
But McGeechan, who has toured with the Lions twice as a player and five times as a coach, insists the decision can not be made too soon.
"Some players have come through last season, but the second season often challenges them in a different way," he said.
"We need to see which of them are really able to kick on. There are a lot of unknowns when it comes to selecting the squad, and it's the same with the captaincy.
"There are people like Chris Robshaw of England coming through and it will be interesting to see how he shapes up with another 12 months under his belt.
"Sam Warburton is an obvious choice and he already has a relationship with Warren.
"You need to see how someone is playing, his demeanour and rapport with players and support staff. You have to be disciplined and make a late call on it.
"In 2009 we didn't make the call until late April, which gave me the time to speak to all the international coaches.
"It also gave me the chance to speak to Paul O'Connell and spend some time with him. He was superb as our captain in 2009.
"But Warren won't be afraid to talk to the contenders and make the call he needs to."
McGeechan believes Gatland is the ideal candidate to lead the 2013 Lions thanks to his successful time as assistant coach in South Africa three years ago.
"Warren is the right choice. He had great involvement in 2009," said McGeechan, who is working in an advisory capacity to the 2013 Lions.
"He's a New Zealander but understands and has huge respect for the Lions ethos as well as anyone.
"He's played against them himself and the 2009 tour taught him what it means to be involved with them.
"He's coached in Ireland, England and Wales and on the last tour he was outstanding. He will be good for continuity.
"He's a great personality to draw players and management together, which is crucial for a head coach. He has a great rapport with players."
The tour ends with three Tests against the Wallabies, who won last month's series against Wales 3-0, and McGeechan believes the Lions are capable of prevailing.
"Wales showed that if you put out a well-balanced Lions side against Australia it should be a very competitive series," he said.
"It's important to allow different combinations time to play together in the build up and you do that by taking fit players."
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