The Whites deservedly led 1-0 at the interval at Tottenham Hotspur through Dan James's goal, which came after a brilliant assist from Jack Harrison.
However, Spurs improved immensely in the second 45 minutes to earn victory thanks to goals from Pierre-Emile Hojbjerg and Sergio Reguilon.
The result means United have won just two of their 12 league games this campaign. They sit 17th on 11 points, two ahead of 18th-placed Burnley.
Phillips felt that a lack of concentration and poor game management cost Leeds but given their first-half performance and who they were missing - Raphinha, Rodrigo, Patrick Bamford, Luke Ayling, Robin Koch, Jamie Shackleton and Crysencio Summerville were all absent at Spurs - the England international feels the Whites' fortunes will turn.
"In the first half we played some good football. We played at our tempo and right at the end [of the first half] we got our goal," said Phillips.
"We went in at half-time really confident. In the second half we knew they were going to come out and want to press us - they did that well. We couldn't really play the same way as we did in the first half.
"We're very disappointed. It's just a lack of concentration in some parts [of the game] that hurt us. We needed better game management. We were 1-0 up in the second half trying to push for a second goal.
"There were times where we couldn't really play out and I think in those moments you probably need to kick it long and soak up the pressure. There were times we did that well but, like I said, it's probably a lack of concentration that has cost us.
"We are missing some very good players in our team, some very important players. We're missing Pat Bamford and Luke [Ayling]. Rapha was out with Rodri too. It's not an excuse, we did come here and play some good football.
"Everyone looks at Rapha as a special talent. I think Jack Harrison did well and caused some problems. I think we needed that. When Rapha misses a game somebody needs to take on the responsibility and I think Dan [James] and Jack did that.
"I'm not one for panicking. The way we play football we'll eventually stack some more points on the board. We're a very fit team. I think the results will come. We just need to be more decisive and concentrate a bit more."
The injury problems saw Joe Gelhardt earn his first Premier League start while Phillips was deployed at centre back by manager Marcelo Bielsa.
He did an excellent job of keeping Harry Kane quiet but admitted he was surprised by the decision to play him in defence.
"To be honest I don't know why he did it," said Phillips of Bielsa's decision to put him in a back three with Liam Cooper and Diego Llorente.
"We wanted to be more defensive with their attacking threat of both Harry [Kane] and Son. So we had Diego [Llorente] on Son and me and Liam Cooper went man for man marking.
"I think it worked well in the first half and even in parts in the second we did well against Harry."
ANALYSIS -Tottenham Hotspur's wingbacks play key role as they condemn Leeds United to defeat.