Manchester United say they cannot "completely eliminate" the possibility of away fans obtaining tickets in areas designated for home supporters at Old Trafford.
More than 3,500 Rangers fans were in the away end for Thursday's Europa League game between the sides but television footage showed dozens more celebrating an 88th-minute equaliser in a game they lost 2-1.
United say they took a number of "strict" preventative measures to block away fans securing tickets in the home end and the club insists tickets were only allocated to supporters on the club's database.
But the club acknowledged that, given the "unauthorised touting of tickets", they could not "completely eliminate" the possibility of away fans sitting in the home end.
United will launch a full investigation into how tickets for home areas reached Rangers fans and say they will impose the "strongest possible sanctions" against registered ticket holders found to be in breach of regulations.
Following the match, Greater Manchester Police said they made 39 arrests across the night - 26 of which came following disorder in the city centre.
The club apologised to supporters in 2023 after more than 2000 Galatasaray fans were allowed to access the home section for a Champions League match at Old Trafford.
The Turkish club were fined £44,420 for the behaviour of their supporters at the stadium.