Three United fans received minor injuries after the incident at a restaurant on Dluga Street in Gdańsk on Tuesday night.
A statement issued by the club on Wednesday read: "Club staff were assisting a number of fans in Gdansk last night and today after an incident where some of our supporters were attacked outside a bar in the city."
Aleksandra Dulkiewicz, the mayor of Gdansk, condemned the attack on Twitter and said: "Yesterday there was a hooligan attack on the fans of the teams that came to the Europa League final.
"Gdansk is an open city. There is no place for any kind of violence! Dear fans and guests, you are always welcome and we cordially welcome you to our city!"
United have distributed 2,000 tickets to supporters for the final although more are expected to travel.
The majority will arrive on Wednesday although some made the trip earlier.
With Poland on the UK government's list of amber countries, supporters attending the game will be expected to quarantine for 10 days on arrival home under COVID-19 regulations.
United are looking to claim their first major trophy in four years since winning the Europa League under Jose Mourinho in 2017.
Ole Gunnar Solskjaer is also in search of his first piece of silverware during his managerial spell at Old Trafford and said the final will define United's season.
"You go into every game as the next one is the most important one," he told a news conference in Poland on Tuesday. "This now is a final that happens to be at the end of a season.
"You can define the season after tomorrow. So far we've improved but we'll only all go home happy if we win."