Barcelona coach Xavi says Manchester United have "grown a lot" under manager Erik ten Hag.
United have drawn Barcelona in their Europa League playoff tie after the Spanish side failed to progress from the Champions League group stage for the second season running.
There is immense history between United and Barcelona dating back to their European Cup Winners' Cup meetings in 1984 and the teams faced each other most recently in the 2019 Champions League quarter-finals.
Xavi made his Champions League debut at Old Trafford under Louis van Gaal in the pulsating 3-3 draw in September 1998 and last year returned to his boyhood club to replace former manager Ronald Koeman.
Barca dropped into the Europa League from the Champions League group stage last year and were paired with Napoli. Xavi agreed they had drawn the toughest team at the same stage again.
"We will have to compete," Xavi said at a press conference. "Going to Manchester against a really tough rival and a historic side that has grown a lot with Ten Hag, with great individual players.
"Yes, it is the toughest rival, once again, in the Europa League. We will have to compete, it's a massive challenge for us, our players will probably be more motivated against a strong team and we will have to compete."
There is added intrigue to the tie as Barcelona midfielder Frenkie de Jong is set to travel to Manchester after resisting a move to United in the summer.
United agreed an €85million fee with Barcelona but De Jong's deferred wages proved to be an insurmountable stumbling block, while the player publicly expressed his preference to stay in Catalonia.
Former United defender Gerard Pique announced his retirement from the game last week and played his final game for Barcelona on Sunday.