Amarilla's fourth career goal kept Minnesota (2-0-2, 8 points) unbeaten despite scoring just their fourth goal of the season. Amarilla scored for the second straight week.
San Jose (0-3-1, 1 point) dropped its second straight despite maintaining possession for 62.5 percent of the match. The Earthquakes registered six shots but did not get a shot on target and were blanked for the second straight week.
Minnesota goalkeeper Dayne St. Clair drew his second straight start. He was named MLS Player of the Week following his eight-save performance. This time, he saw little activity and notched his seventh career shutout while making his 20th career start.
San Jose goalkeeper JT Marcinkowski finished with four saves as the United took 13 shots overall.
After Minnesota's Emanuel Reynoso missed an early scoring chance when his left-footed shot sailed wide, the United finally broke through thanks to Amarilla.
Amarilla tallied when he got a long cross from Robin Lod and split two defenders in the center of the box. He finished the play by pushing a left-footed shot over the left hand of a diving Marcinkowski.
Amarilla nearly scored again four minutes later but Marcinkowski made a point-blank stop on his right-footed shot.
Minnesota nearly took a two-goal lead in the 56th minute but Bongokuhle Hlongwane's right-footed shot from outside the box was stopped. The United kept getting scoring chances but Marcinkowski stopped Hlongwane in the 64th minute and Lod a minute later.
Minnesota missed another chance at doubling the lead in the 81st minute when Lod's left-footed shot from the right side of the box sailed slightly to the right.
San Jose had five free kicks in the final 10-plus minutes but never came close to breaking through.