The All Blacks and Lions played out a thrilling 15-15 draw in the third Test on Saturday to see their series finish locked at 1-1.
Spencer felt the tourists were capable of becoming just the second Lions team to win a series against the All Blacks, if they had more time in the lead-up.
“My honest opinion is yes,” he told a news conference on Sunday.
“I think that preparation is extremely important. Gold medals are won on the training pitch, long before they are won on the rugby pitch.”
A fitting finale to an amazing Test series @AllBlacks.
— British&Irish Lions (@lionsofficial) July 8, 2017
While Spencer would like more preparation time, there have been calls for shorter Lions tours.
The former England captain, 69, said there was no reason to reduce tours, with the Lions having played 10 games in New Zealand.
“I do think we really need to sit down, all the stakeholders, including perhaps ex-Lions who know what the Lions is about. They get the concept of the Lions,” Spencer said.
“It’s not beyond the wit of man to come to some sensible agreement just for a couple of weeks every four years and that’s what I would encourage people to do.
“I think the players now have put the Lions in a very special place and we don’t want to lose that, we want to progress that, we want to encourage it, we want to inspire people to take part in the Lions.”
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