The Eredivisie champions get their Group B campaign underway on Tuesday with a daunting trip to the Catalan giants, who have enjoyed a 100 per cent start to the season.
Van Bommel - who spent 2005-06 with Barca - has demanded that his players remain professional and only think about getting their hands on Messi's shirt at the end of the game.
"If tomorrow we get a good result I don't care if they fight for Messi's shirt," he told a news conference.
"That is part of being a professional, to always be concentrating. I know they will ask him for the shirt, but they shouldn't do it at half-time."
Van Bommel returns to Camp Nou but, while there are few familiar faces remaining, he hailed former team-mate Messi.
"Only Leo remains in the team from my year here," the PSV boss added. "It was in 2005 and you could see he would be one of the greatest.
"I think he is still the best in the world despite not winning the Ballon d'Or."