The Scots have so far failed to secure a victory in a dismal campaign, which has seen them collect just two points from six games.
Home and away meetings with Croatia, as well as the visit of Belgium and a trip to Macedonia, still lie ahead.
Naismith told BBC Scotland: "We've got very tough games but the players are playing for their future international careers.
"I said earlier in the week that I think we've got to grow up in terms of digging in and being mature about when we need to sit in and soak up the pressure and when we need to expand and try to go for the goals.
"As players, that's what you need to do or you're going to find yourself out of the national team."
Despite further defeats, against Wales and Serbia, Naismith disagreed with former Wales winger Mickey Thomas' claim that the current crop of players are the "worst Scotland team ever".
The forward said: "The statistics don't lie but I think we are a better team than what he was saying.
"To be honest, I don't think too many in the dressing room are paying too much attention to what a Welshman is saying about the Scottish national team."
A second-half double by striker Filip Duricic sunk the Scots on a depressing night in Novi Sad.
Naismith said: "In terms of overall play, it was a hard night on the pitch.
"Both teams did as well as they could, the difference was in both boxes.
"We didn't take our chances and we probably gave up easier goals than we would like to be conceding.
"If you look at the team overall, it was maybe a bit of inexperience.
"We got to grips with it and, as the game went on, we gave more than we did in the first 20 minutes. Then, we're in their box, they make the save and they go up the park and score.
"For both sides, it was a tough pitch to create chances and take them. But I think with the defensive side of things we won't be happy."
Charlie Adam agreed that the hosts were simply more clinical in front of goal on the night.
The midfielder said: "In the end, they took the chances they had.
"We had a couple of chances in the second half that we probably could have done better with.
"We are bitterly disappointed that we didn't get anything out of the game because I thought we played well.
"The pitch was bad but we can't make that an excuse. It was the same for both teams.
"We are disappointed with the results we've had in the last two games and it's just been a bitterly disappointing campaign."