The Magpies new signing scored in their 3-3 draw with Crystal Palace in their last Premier League game, and he will be hoping to continue to improve following his switch from Feyenoord this summer.
The 25-year-old will come up against his former boss Ronald Koeman when Newcastle travel to Southampton this weekend, and the Dutch defender has backed the Saints manager to be a success in the Premier League.
“He’s a good trainer. It'll be nice to see him in the Premier League and I'm looking forward to playing against him," Janmaat said.
“I think he is going to do well at Southampton. It takes time because they lost their manager and a lot of good players and it’s not easy to win games and play around when there’s been a lot of changes. I’m sure he’s going to do well because he’s a great trainer.”
The Magpies are still looking for their first win of the new season and a trip to the south coast for Saturday's game will not be an easy fixture for Alan Pardew's team.
Southampton won 3-1 at West Ham in their last game and Koeman will now look to kick on and pick up his first home win in the Premier League.
The Saints beat Newcastle 4-0 at St Mary's in March and Alan Pardew will be desperate to reverse their fortunes this weekend, despite their slow start to the new campaign.
But Janmaat believes that Newcastle can still finish high enough in the division to earn a place in Europe next season.
“I don’t know what we can do, but I think we have to focus on every game. We can do well.
“We have a good squad and we have to take each game as it comes and at the end of the season we’ll see where we end up.
“We want to play in Europe. I don’t know if it’s possible this year but we want to and we will do everything to make that happen.”
Will Newcastle beat Southampton this weekend? Can the Magpies earn a place in Europe next season?