The 20-year-old, who spent last season on loan at Hull City, has performed as a full-back for United in pre-season and is understood to be in line to replace Patrice Evra should the Frenchman become injured.
"On the first day back in pre-season, the gaffer put me into left-back and I had a feeling that was the way he was thinking for me," he told reporters.
"I had to learn quickly but I'm very comfortable playing at left-back. I'm not a bad tackler, decent in the air, but maybe I have to work on my positional sense as a defender. The manager was happy, though.
"We'll have to wait and see what happens between now and the start of the season next week, but they've no real cover for Patrice Evra. I think I'm first in line if anything happens to him, unless they sign someone in the meantime."
United travel to Everton in their Premier League opener on Monday.