who was supposed to mark mikel merino

The best-supported answer is: nobody was really supposed to mark Mikel Merino for stretches of that match, which is exactly why Arsenal’s setup caused problems. One report on Arsenal’s use of Merino as a false nine says Spurs were left with “no one to mark” him for extended periods.

What that means

Merino was drifting deeper than a normal striker, so defenders had to choose between stepping out or holding their line. That movement created uncertainty, and the article describes the opening-goal buildup as coming from him finding space between the lines.

In plain terms

• He was not playing as a classic center forward.

• Spurs’ defenders did not have a clear man-marker assigned to him for long spells.

• The tactical issue was more about space and rotation than one named defender failing a job.

Context

So if the question is being asked as a specific “who was on him?” debate, the answer is probably “no one consistently,” rather than a single player. The source frames it as a collective defensive problem caused by Merino’s movement.