Manchester United's Fletcher is back playing regularly after missing a lot of the past few years due to a chronic bowel condition and Everton striker Naismith is backing him to step straight into the vacancy left by Celtic skipper Brown.
Brown is out until October with a hamstring injury meaning he will miss Scotland's opening 2016 European Championship qualifier against Germany on Sunday but Naismith is confident Fletcher is the perfect replacement.
"It's great to have Darren back. He is a person who brings the best out in others," Naismith told Sky Sports.
"Without being a very loud character he leads by example and he makes everybody feel welcome. Young players coming into the squad, he is great with them as well.
"But as a player, he has had a great pre-season. He's been playing week in, week out for Man United, and getting back to his best. For Scotland, that is a massive asset to have.
"It's a blow missing Scott Brown but Darren is certainly a fantastic replacement for him."
Scotland meet World Cup champions Germany at the Westfalenstadion in Dortmund on Sunday in their first Group D qualifying match for Euro2016.
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