Boli is Al Rayyan's fifth foreign player along with Algerian Yacine Brahimi, Cameroonian Franck Kom, Argentine Gabriel Mercado, and Korean Lee Jae-Ik.
Al Duhail now are just two points ahead of second-placed Al Rayyan.
Pierre-Michelle Lassoga, Mohammed Salah El Neel and Morteza Pouraliganji scored for Al Arabi.
The Portuguese coach expects Mandzukic to solve his goal-scoring problems.
Mohamed Benyettou and Omar Ali scored Al Wakrah’s goals, while Hassan Al Haydous reduced the margin for Al Sadd late in the game.
Croatian striker Mario Mandzukic was officially presented before the media on Thursday.
Al Sadd are fourth in the standings with 18 points from nine games, 11 behind leaders Al Duhail who have played 11 games.
Umm Salal had held Rayyan to a 2-2 draw when the team met in the first phase.
Midfielder Assim Madibo is back in training after an ankle surgery.
Unbeaten Al Duhail are four-points ahead of second-placed Al Rayyan and 11 in front of Al Sadd, who have two games in hand.
The 33-year-old Croatian striker has played for European giants such as Bayern Munich, Atletico Madrid and Juventus.
The first semifinal between Al Duhail and Al Sailiya will be played on Friday, 10th January, and second semifinal featuring Al Sadd against Al Rayyan on Saturday, 11th January.
The meeting was attended by Ahmed Salman Al Adsani, Director of Competitions Department at QSL, Abdullah Al Balooshi, representative of the Qatar Football Association’s Referees’ Committee, and representatives of QNB Stars League clubs.
Umm Salal are looking to improve their squad after a string of losses saw them fall to the bottom of the table.
Slovak midfielder Weiss, who had a successful career with Al Duhail, joined Al Gharafa in 2016.