It threatened to mar what had been an evening of celebration for Erik ten Hag's men, who celebrated a comfortable 4-1 victory over the Spanish opponents to put one foot in the Europa League quarter-finals.
Responding to their 7-0 humiliation at the hands of Liverpool, United star Marcus Rashford fired in his 26th goal of the season after just five minutes before Ayoze Perez equalised on the half-hour mark.
But United found another gear in the second half with Antony, Bruno Fernandes and Weghorst all scoring to record an impressive win over the La Liga side. They will now travel to Seville for the second leg with a healthy three-goal cushion for the return leg.
It remains to be seen whether UEFA will decide to act over the fan trouble inside Old Trafford after the scenes unfolded on Thursday, and whether that could affect Betis fans being at the Benito Villamarin in a week's time.