Dier felt that the decision to award a free-kick that lead to the first goal in the game was totally wrong and believes that everyone could see that it was a massive mistake which changed the game in Arsenal's favour.
Dier feels Mike Dean changed the game in Arsenal's favour
Mauricio Pochettino's side though were also well below par for the game but Dier felt that just when Spurs were starting to control the game, the free-kick decision that lead to Shkodran Mustafi's goal was the difference.
Speaking to BBC Sport after the game, Dier said "that there is so much on the line" in a derby match and therefore for "a goal to be decided" like the opening goal was is "very disappointing" as it was clear to see that it was a fair tackle according to the midfielder.
Dier continued to say that the decision "changed the game" as from there Arsenal were in the ascendancy but the midfielder said that he doesn't want to "get in trouble" for saying to much but felt it was clear in a way that "everyone can see it."
Spurs must bounce back quickly or they can forget about the Premier League title
The midfielder added that if the foul on Alexis Sanchez is a foul then "soon we won't be able to tough each other" which he knows for sure that "nobody wants that" but at the end of the day "there can be no excuses" for the under par performance from the whole Spurs team on the day.
The result against Arsenal leaves Spurs title challenge in the ballance as they are now 11 points behind leader Manchester City but the key for the team now is to bounce back strongly at home to West Bromwich Albion next week if they want to get back on track in the Premier League.