The five-foot-seven English forward will have to sit out Saturday's game against visiting Sporting Kansas City after receiving a caution last weekend in Houston, when he was essentially hurled to the ground by six-foot-four Dynamo defender David Horst during an altercation. Only Defoe saw yellow, his fifth of the season which triggered a one-game ban.
Nelsen's beef is that officials seem to be swayed by simulation and the extras that can follow a foul rather the actual incident.
"He's such a honest player," Nelsen said of Defoe. "Especially in England, you don't dive, you don't fall down, you don't complain. You get that sort of treatment and you have to deal with it.
"But over here, a lot of players fall and dive and roll. And the referees react on that. Where(as) our guys are sometimes a wee bit too honest and they take the punishment. The league talks about stamping out all that play, but just because our guys don't dive, it doesn't mean it's not happening.
"That's the thing that frustrates me the most is that the referees, they react on what happened if it's a dive or it's a roll or something like that, then they react. And not the actual impact itself, which they should be doing."
No stranger to having a target on his back, the 145-pound Defoe has not complained and has shown he can take care of himself, not backing down from anyone.
But the 31-year-old has surprised even himself with the five cautions so far in his first season in MLS.
"Which is unusual for me," he said after the midweek friendly with Tottenham. "I don't normally get involved, I don't really get yellow cards and that."
Defoe understands, however, that opposing teams are looking to get him off his game.
"Well that's their job at the end of the day," he said after the recent 1-1 tie with Vancouver."You've got to do whatever it takes to win the game.
"Obviously I'm not in their changing room, I don't know what the manager says or what they do, but at the end of the day, for me, I've just go to get into the areas where I'm going to score goals. And obviously they're going to try and do stuff, I suppose, to put me off my game. But it's not a problem. It's important just to keep focused and as a forward make sure when we get the chances, that we put them away."
Going into this weekend's round of play, Defoe was one of 13 players with five cautions. D.C. United midfielder Perry Kitchen led the league with seven yellow cards while six players had six cautions.
Defoe leads TFC in yellows, despite only having played 14 of 17 league games.
Portland's Darlington Nagbe led the league with 61 fouls suffered in 20 games. Defoe was not in the top 10.
But as Toronto's leading scorer, with 11 of the club's 26 goals, he is a key to the attack. Defoe ranks fourth in league scoring.