Jose Mourinho should be sent to prison over his treatment of Bastian Schweinsteiger, according to a member of world players' union FifPro.
Schweinsteiger appears to have been cut out of the Manchester United first-team picture since Mourinho's arrival as manager in the summer, with reports that he has been training with the reserves.
The World Cup winner's name was conspicuous by its absence from United's squad for Wayne Rooney's testimonial against Everton at Old Trafford on Wednesday, a match in which Mourinho used 22 players.
Dejan Stefanovic, a lawyer and FifPro member from Slovenia, said: "It's clear bullying. In Slovenia, we would have indicted Mourinho and asked for the highest penalty - three years in prison.
"This is clearly meant to discourage Bastian. Mourinho is setting a bad example. I would take it to the ordinary court.
"In our case, the penal code is quite clear. If there is bullying and harassment, there's a prison sanction, a maximum of three years in prison.
"Schweinsteiger should turn to the Professional Footballers' Association and he should file a complaint and I would also seek penalty for Mourinho.
"All the players of the first team have to train with the coaches of the first team at the same place, at the same time as all other team-mates.
"That's something that should be applied to each and every club and should be written within FIFA regulations."
United have declined to comment on Stefanovic's remarks.
Midfielder Schweinsteiger, 32, joined United on a three-year deal last summer but failed to make a big impression during an injury-hit campaign.
He nevertheless remains a revered figure in the world game after a glittering career with both Bayern Munich and Germany.
Bayern president Karl-Heinz has expressed his shock at the situation and believes there could be repercussions for United.
Rummenigge said: "I could hardly believe it. One or two players are going to think long and hard in future about whether they want to go to such a club. Nothing like that has ever happened at Bayern Munich."
Schweinsteiger in fact forced his way back into the first-team fray at Bayern after being told in 2005 by then-coach Felix Magath to train with the reserves since he did not have a place for him in the first team.
The midfielder bounced back to inspire Bayern to a league and cup double, contributing to his haul of eight Bundesliga titles, seven DFB-Pokals and the 2013 Champions League title with the club.
He was also a key part of Germany's 2014 World Cup triumph, and won last season's FA Cup with United. Rummenigge added: "It's a lack of respect for a worthy player. It's a method which is strange to me.
"Schweinsteiger is a key player, a world champion, a personality, while at the same time he has an exceptional attitude."
Meanwhile, Wayne Rooney believes United's attacking prospects are good after his testimonial run-out alongside new signing Zlatan Ibrahimovic.
United failed to find the net as they played out a goalless draw with Rooney's former club Everton in their final pre-season friendly at Old Trafford.
But Rooney linked well with Ibrahimovic, who was making his home debut, and the combinations with fellow forwards Jesse Lingard and Anthony Martial were also promising as the team continued its re-tuning under new boss Jose Mourinho. Rooney said: "I think that will come with time, in the next few weeks when you're getting your full fitness.
"I think Wednesday night was a little step forward. A few of the lads felt a bit sharper. In the first half we opened them up at times and the rotation was there between players, little combinations. It was a decent performance at times."