That is according to Manchester United team-mate Anthony Martial.
United’s £89million world-record signing faces off against his older sibling at Old Trafford on Thursday (8.05pm).
Jose Mourinho’s men are preparing for their Europa League round of 32 first leg, with Paul Pogba expected to start.
Florentin is also set to feature from the beginning in Saint Etienne’s defence, having started their last three games in Ligue 1.
Paul Pogba and Anthony Martial may both start for Manchester United against Saint Etienne
They are well known for being big jokers and mickey-takers
Martial knows how much Thursday’s showdown means to his United and France team-mate.
And while strong personalities will shine through before and after the game, the 90 minutes of action will be hotly and seriously contested.
"They are well known for being big jokers and mickey-takers but, once they are on the field, it will be like two warriors who are going all out to win the game," Martial told the club’s official website.
Florentin Pogba is preparing to face his brother Paul at Old Trafford on Thursday
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"If one beats the other, he’ll immediately start giving out loads of stick so they’ll be desperate to do all they can to win!"
Martial will be vying for a starting role too, having shone against Watford on Saturday in United’s 2-0 win.
The 21-year-old was given an opportunity to impress by Mourinho and managed a goal and an assist.