Rashford, who delivered an assist in Barcelona's stunning 6-0 thrashing of Valencia on Sunday, has caught the eye of his German coach since arriving from Manchester United.
"I really follow Marcus (Rashford) since he started with Manchester United. And I always said, 'Wow! What a player he is!" Flick told reporters at Wednesday's press conference in Newcastle.
"He has speed, but also, in one-against-one situations, it's really fantastic. And also finishing, it's unbelievable.
"What I saw from him in the first weeks here, it’s very good. And I think also he still has potential to show us more."
Lamine Yamal, who started the season strongly with two goals and two assists in LaLiga, will miss Thursday's clash due to a hip injury on international duty with Spain. The 17-year-old may also be doubtful for Sunday's match against Getafe.
Barcelona will also be without Alejandro Balde and Gavi, plus long-term absentee Marc-Andre ter Stegen, though Frenkie de Jong is available for selection.
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"We all know that Lamine (Yamal) is not playing against Newcastle, maybe not on Sunday (against Getafe)," Flick said.
"But at the end, we have a strong team, and in his position, we have Marcus Rashford. If someone up front cannot play, he can step up and play like he showed on Sunday against Valencia. That is how good he is.
"I hope that he can show in England the same performance he did on Sunday."
The German coach remains confident about his team's Champions League prospects despite facing stiff competition.
"I'm not thinking about who the favourite is (to win the Champions League this season). There are some very good teams, mainly coming from the Premier League," he said.
"What we saw against Valencia was very positive. We are playing well in all areas. With this confidence, it is important to play tomorrow. They are very strong, but we are confident."
