The 18-year-old Barcelona B forward stepped up for Saturday's friendly and looked at home among his senior team-mates, scoring twice and grabbing an assist.
Having opened the scoring, the Spain Under-19 man turned creator for Sergi Roberto to double Barca's lead inside 10 minutes.
Luis Enrique's side were in rampant form and El Haddadi headed his second goal of the game with 17 minutes gone, with Gerard Pique making it four shortly afterwards.
Further goals followed after the interval as Marc Bartra headed home and Sandro Ramirez capped the performance with a well-taken drive.
Yet Munir's performance will have caught coach Luis Enrique's eye as he looks to mastermind an upturn in fortunes for Barca following last season's disappointments.
El Haddadi headed Rafinha's cross in via Tornes' right-hand post for his second of the afternoon and when Pedro worked the ball into Pique to tuck home, Luis Enrique's side looked on for a big score.
The remainder of the first half passed by sedately, but Barca emerged revitalised after the break and Bartra headed half-time substitute Ivan Rakitic's free-kick beyond replacement HJK keeper Carljohan Eriksson.
It eventually arrived in the 81st minute as Sandro latched onto Rakitic's measured throughball and lashed a fine strike across Eriksson.
