The Parkhead outfit suspended 128 fans from their games and relocated 250 season-ticket holders following damage caused to seats at Fir Park.
Smoke bombs and flares were also thrown during Celtic’s 5-0 Scottish Premiership win.
Scottish Professional Football League (SPFL) chief executive Neil Doncaster described the scenes as “shameful” and promised an investigation once it receives its match delegate’s report.
Under SPFL rules, Celtic will have to pay for the damage if found guilty,
Mjallby said: “It is very unfortunate what happened and I don’t think it left the club with any other option but to suspend some fans.
“You don’t want to have mayhem, you can’t have this disruption inside sports grounds so we have to get together to support the club in the best way. It is a distraction I don’t like to see because I like to work with the players. The club has reacted in the best way,” he added.
Skipper Scott Brown returns to the squad for tonight’s clash at the Nou Camp after missing the last three European games through suspension following a clash with Barca star Neymar at Parkhead, while fellow midfielder Nir Biton has served a one-game ban.
Mjallby said of Brown: “He hasn’t got a point to prove to us, he is one of our best players, he is very important and we have missed him a lot.
“But maybe for himself and the public, he can prove that he can play at this level.
“The target is to finish off this campaign on a good note.
“We are disappointed that we don’t have a chance of European football in the new year but it is a hard group and there is not much we can do now.
“It is a good chance to take on more experience at this level.”
Like Biton, forward Georgios Samaras also missed the trip to Motherwell with a virus but is fit again.
Tom Rogic (calf) and James Forrest (hamstring) along with Beram Kayal (foot) and Steven Mouyokolo (Achilles) remain out.
Defender Adam Matthews has returned to training following shoulder surgery and Mjallby said: “He has got a chance. He has been out for seven weeks and it is great to have him back in the squad but we have to see how he feels.
“It is a big ask to throw him in against Barcelona.”
The Catalan giants are without forward Lionel Messi, goalkeeper Victor Valdes and midfielder Cesc Fabregas.
Brazil full-back Dani Alves is suspended but Spain midfielders Andres Iniesta and Xavi Hernandez could appear for the hosts after shaking off injury problems.