Fabio Cardoso says below-par defending cost Rangers all three points in their 2-2 draw at Partick Thistle.
The Ibrox side took the lead through Alfredo Morelos but went 2-1 behind after their back-line's error-strewn start to the second half.
Graham Dorrans earned Pedro Caixinha's side a point and Cardoso was in no doubt as to what cost his side.
"We committed a lot of defensive mistakes and we have to improve," the centre-back told Rangers TV.
"It was a hard game and we can't commit mistakes like this. We scored two goals and with that we can't be losing the game.
"After we conceded the second goal we tried and tried to score. We got one but we needed two. But again, when we score two goals, we can't lose the game."
Cardoso gave away a free-kick that led to Blair Spittal's deflected 50th-minute equaliser on Friday night, before the visiting defence switched off five minutes later and allowed Chris Erskine to tap home the Jags' second.
Erskine was sent off for a reckless challenge soon after, before Dorrans drew his side level with a curling effort into the top corner.
Cardoso, 23, admits Rangers have been guilty of making slow starts to the second half of matches this term, and is determined to remedy that ahead of Tuesday's League Cup quarter-final tie away to Thistle.
"It is something we have to improve, we have to control the game because teams start very strong against us," the Portuguese said.
"We have to start stronger and we have to improve for the next games.
"For us, the game could be tomorrow. We need to play fast, to improve and win.
"We are more than we showed today."
A first-half injury to Lee Wallace meant recent loan signing Declan John made his debut at Firhill.
While he was glad to get some game time, the Welshman, who ended the match with "an ankle niggle", says there is more to come.
"I haven't played a game in two to three weeks so it was tough, but I did enjoy it," John told Rangers TV.
"With the football we play, it suits me down to the ground. Defending-wise, I do need to work on stuff but going forward, I have got that in my locker.
"It's tough, when you go 1-0 up you obviously don't want to concede a goal, but it was good that we came back and got the second goal."