Reijnders delivers early Christmas present for impressive AC Milan
Milan are six points behind leaders Inter Milan who face Lazio later on Sunday, in large part thanks to a stellar display from Reijnders who set his team on the way to victory with his third-minute strike.
The Netherlands midfielder collected Christian Pulisic's pass just outside the area and then set off on a mazy dribble past five Monza defenders before poking his first home goal for Milan past Michele Di Gregorio.
Building on their midweek win at Newcastle United which ensured a spot in the Europa League knockouts, third-placed Milan moved up to 32 points.
"It's an honour to play here of course, to score here at the San Siro is very nice," said Reijnders, who believes Milan can still fight for the league title.
"Of course, you always have to believe, we are not finished yet. We are now halfway through the season and we have to keep going. Today was a good start."
Also on target for Stefano Pioli's team were teenage debutant Jan-Carlo Simic, who netted his first Milan goal shortly before the break, and Noah Okafor whose 76th-minute strike came following more fine play from Reijnders.
Simic 'dream'
Born in Germany, centre-back Simic represents Serbia at youth international level.
The 18-year-old was brought on midway through the first half for Tommaso Pobega, the latest in a long line of Milan players to suffer a muscular injury this season.
And he doubled the hosts' lead three minutes before half-time, ramming home Rafael Leao's low cross before charging off to celebrate.
And the San Siro faithful then began chanting Simic's name as his parents both wept with joy in the stands.
"I'm pleased for them... I'm grateful for how they've help me, even in difficult times," said Simic to Sky.
"It's a dream for any kid to make your debut for a club like AC Milan and scoring on your debut is incredible. But I've worked very hard for moments like this."
Milan are seven points ahead of Roma and Bologna who are just outside the top four and face off later on Sunday, one day after the first anniversary of Sinisa Mihajlovic's death.
Mihajlovic began his long playing career in Italy at Roma in 1992 while his last job as a coach before succumbing to leukaemia was at Bologna.
Inter are one point ahead of Juventus in the title tussle and can further pull away from the chasing pack with a win at Lazio in Sunday's late match.