Full-backs:

Right back: Hector Bellerin
Bellerin's injury comes at a most unwelcome time for the Gunners as the Spaniard is one of the key cogs in Unai Emery's setup. One of the fastest players in the Premier League, Bellerin walks into this squad based on his overall ability, and the lack of a settled right-back at Manchester United.
Left-back: Luke Shaw
The young English full-back Luke Shaw is one of the easiest selections in the XI. After returning from a long-term injury and going through a rough patch with United's former boss Mourinho, he is finally living up to his potential.
Shaw has been one of the keys for United all season and has displayed incredible consistency, both at the back and going forward. Shaw is one of the success stories this season and will be determined to put in a strong performance against Arsenal.