The England forward has recovered from the flu that kept him out of Saturday's 4-0 La Liga win over Athletic Bilbao and is available for Tuesday's meeting with Chelsea at Stamford Bridge.
Four of Rashford's six Barca goals so far have come in the Champions League, including two on the team's previous visit to England to face Newcastle in September.
The move to Spain has helped Rashford, 28, resurrect a career that had stalled badly at Old Trafford.
"I'm happy to have him in the team, playing for Barca," Flick said on Monday. "I've followed him throughout his whole career and I've always been impressed by his quality and what he can do in the box, and he's showing that in Barcelona.
"That change for him, new football, in Barcelona, with good weather. For me it's incredible to see how he's always smiling. That's the atmosphere we have and it's very good for him."
Barca and Chelsea are level on seven points after four matches in this season's Champions League as they seek direct qualification to the last 16, which requires a top-eight finish.
Flick's task will be aided by the return from injury of Raphinha, who made his first appearance in almost two months as a substitute in Saturday's win.
"Step by step with Raphinha," said Flick. "For me, the important thing is that he's back. About the minutes and how much he will play, we'll decide.
"He's had the same injury twice and that's not good at this stage, so we'll be careful.
"I'm very happy that he returned because as soon as he played on Saturday, we saw the difference. The dynamism, he created spaces and I'm very happy to have him."