Bella-Kotchap, who joined the Premier League club this season, had spent four years at VfL Bochum and is an Under-21 international.
"He has really developed well at Southampton," Flick said on Thursday.
"We talked to our Under-21 team where he is an important player.
Flick said the player's performances had also been a surprise for Germany's coaches.
"That is why we will use these eight days to have a look at him," Flick said.
There were no other surprises in his squad that will host Hungary in Leipzig on September 23rd before travelling to England three days later.
Timo Werner (26) and Jamal Musiala (19) will lead their attack, along with Thomas Muller (33) and Leroy Sane (26).
Germany are in second place in their Nations League Group Three, a point behind Hungary and four ahead of England but their focus is already firmly on the World Cup.
The Germans, four-time winners, crashed out in the first round in the tournament in 2018.
"This is the dress rehearsal for the World Cup and already here we have little time to prepare for the World Cup so we have to get into tournament mode," Flick said.
"We won't have a lot of time to get into that mode in Qatar and we will have to hit the ground running."
