A tactic that worked impressively - despite the narrow defeat - in the second-leg tie between the two sides last season was the use of midfielder Mikel Merino as a number nine. PSG, known for their ability to press intensely and often in a man-to-man fashion, stepped up to Arsenal as they looked to build out from the back. A key principle of positional play is finding the free man. When opponents apply man-to-man pressure, finding a free man is harder. With Merino dropping deep into central midfield, PSG centre-back Willian Pacho was reluctant to follow him, which allowed the French team to keep an extra player in the defensive line. Arsenal, without a striker, however now had an extra man in midfield. PSG's midfield trio, alert to Arsenal's midfield three, looked to shift across onto Merino at times but this would leave another Arsenal midfielder free helping the Gunners get up the pitch.…