Amid indications this week from sources close to the club that Messi is poised to sign a contract extension that would keep him in Miami at least through next year, the eight-time Ballon d'Or winner showed just why the club and Major League Soccer are so keen to see him stay.
The 38-year-old now has 22 goals in 22 matches played this season. With his seventh double of the MLS season, he moved to the top of the league scoring list, passing England's Sam Surridge, who has 21 for Nashville.
"It was another ordinary night for him, which is totally extraordinary for any other football player," Miami coach Javier Mascherano said.
"His ability not only to score but to make the team play, to find situations, and to carry the team on his shoulders when we needed him most and faced difficulties - that is the advantage of having him, and today we were able to take advantage of it."
Miami dominated from the outset on a rainy night at their Chase Stadium in Fort Lauderdale, Florida, against a team already eliminated from playoff contention.
They took the lead in the 35th minute when Messi produced a well-judged through ball forward to Tadeo Allende, who beat United goalkeeper Luis Barraza with his first touch and smoothly fired into the right corner of the net.
Christian Benteke headed the visitors' level in the 52nd minute.
Messi forced a pair of quick saves from Barraza before he fired Inter back into the lead in the 66th minute, collecting a pass from Jordi Alba in the box and turning to slot a left-footed shot past Barraza.
Two minutes after coming on as a substitute in the 69th minute, Argentine Mateo Silvetti was fouled in the box by Barraza.
The referee pointed to the spot and the 19-year-old stepped up to take the kick, but he smashed the attempt off the crossbar.
"Leo gave it to Toto (Silvetti) surely as a gesture to let him have his first goal, which I think is something very good, unfortunately he couldn't score it," Mascherano said, adding that the gesture was one Messi had made often in his career to "involve someone who needs it more than him."
But Messi pushed Miami's lead to 3-1 with a bit of trademark magic in the 85th, picking up a pass just outside the area and eluding his defender to loft a shot out of the reach of Barraza.
It proved invaluable, as Jacob Murrell struck for DC United in the seventh minute of stoppage time to trim the deficit to one.
With the win, one Mascherano called "very important," Miami climbed one spot to fifth in the Eastern Conference with 52 points. But they'll have to wait to secure their playoff berth after the New York Red Bulls produced a 2-0 victory over Montreal.
Check out the full match stats between Miami and DC United here.