Tom Curry's (25) red card after an upright head-on-head collision in the third minute of Saturday's 27-10 World Cup victory over Argentina earned him a two-game suspension and followed bans on Owen Farrell (31) and Billy Vunipola (30) for dangerous tackles.
Curry and Farrell will not be available for Sunday's match against Japan but Sinfield, England's defence coach, said everyone was working hard to eliminate head contact.
"It makes life really difficult for the players, first and foremost," Sinfield told reporters on Wednesday.
"We all want to see the players out on the field, we want to see the best players in the world go head to head. We’ve just got to be careful.
"We continue to work on our tackle skill – and work incredibly hard. The guys have bought into it and have done so for some time, but they’re human and they make mistakes.
"Unfortunately, we’ve had to deal with four red cards in six games (one was rescinded). We’re getting pretty good at defending with 14 men, but we want to have our full complement on the field for as long as possible."
Sinfield pointed out that England gave away only seven penalties on Saturday and said they don't have a discipline problem.
One problem they do have, however, is scoring tries. They did not get remotely close to the Argentina line, though they will argue they had no need to such was their dominance in the middle of the park which enabled George Ford to kick six penalties and three drop goals.
A World Rugby statistical review after the first weekend of games noted that England made one line break - the lowest of any team playing - two offloads and 76 passes - both figures also among the lowest.
With the greasy ball, their one-man disadvantage and their control of the contact area, those figures are not a huge surprise as England quickly worked out what was working.
More surprising is that they missed 23 tackles, though the match did not feel like they were being run through, as it did when they missed 27 in the final warm-up defeat by Fiji.
Sinfield was unconcerned.
"The numbers don't have emotion, passion, they don't understand particularly the relationship between a captain and the referee," he said.
"The analytics are important, we try to understand why certain things have happened, but I like to think as a coaching group we trust the players and we back them to make decisions."
England name their team to play Japan on Thursday.
