The Reds were beaten 3-1 by Real Madrid in the Champions League on Tuesday night, with Alexander-Arnold at fault for Los Blancos' second goal.
The 22-year-old was flummoxed by Vinicius Junior's movement throughout the first leg of the quarter-final tie, while his performance will have further hindered his hopes of making the England squad for this summer's Euros.
"The area he can improve would be his feet patterns - movements which allows him to turn and match his opponents run or dribble," wrote the 40-year-old on his Twitter account.
"If you can't move them quick enough, it doesn't matter how fast you are - you will get beat. Foot patterns will get you in defensive positions to turn quick."