Liverpool captain Steven Gerrard has claimed that it is common for footballers to try "every trick in the book" in order to gain the upper edge in games.
Chelsea's players were criticised for surrounding referee Bjorn Kuipers during their Champions League exit against Paris Saint-Germain on Wednesday.
Gerrard told BBC Sport: "I think it's normal when you play games at that level. Players want to win so much.
"You've got managers who want to win so much, players will try every trick in the book to get over the line and try and win football matches.
"Sometimes it doesn't look nice, but we've all been guilty of it throughout our careers, of not always abiding by the rules."