The Manchester United historian has confirmed that prior to Mohammed Salah the last players to score hat-tricks for away sides at Old Trafford in the League were Dennis Bailey and before that Kevin Mabbutt.
Manchester United 0 v 5 Liverpool, Premiership Matchday 09, 24th October 2021, Season 2021-2022, Venue:Old Trafford (mufcinfo.com)
Mohamed Salah 1st player to score a opponent hat- trick at Old Trafford since Read Madrid's Ronaldo 12', 50', 59' on 23/04/2003, last player to score an opponent hat-trick at Old Trafford in a League match was QPR's Dennis Bailey 5', 58', 86' on 01/01/1992.