Sergio Aguero's goal gave Guardiola's men the lead at half-time, but the Argentina international missed two good second half chances
Pep Guardiola acknowledged Manchester City need to develop a more ruthless instinct after they were held to a frustrating 1-1 draw by Middlesbrough.
Guardiola's side made a major statement on the European stage when they beat Barcelona 3-1 in the Champions League on Tuesday.
But four days later City looked increasingly lethargic after an impressive first half at Eastlands.
Sergio Aguero's 150th City goal gave Guardiola's men the lead at half-time, but the Argentina international missed two good second half chances to add to that tally.
City winger Kevin De Bruyne also put the ball over an unguarded net and the hosts were made to pay for their profligacy in the first minute of stoppage-time when Marten De Roon headed in his first Middlesbrough goal from George Friend's cross.
"We need to improve in both boxes, both offensive and defensive, to score goals and not concede," Guardiola said.
"In the second half, we didn't play like the first one but I think they had one chance at the beginning of the second half, and another in the last minute.
"Maybe in the last minutes, the team can be tired. If we scored the second goal, the chance for Sergio, the game was over. In the last minutes, anything can happen."
The result means that, for the third successive league match, City have drawn 1-1 after dominating, having suffered the same fate against Everton and Southampton.
"When you see all three games, we dropped six points, but they arrived maybe four or five times in our box," said Guardiola, whose side have surrendered top spot in the Premier League to Chelsea.
"We attacked really well and controlled the counter-attacks and we created enough chances, more than enough to win the game.
"But when you arrive in the last minutes, and it is just 1-0, everything can happen."
Middlesbrough manager Aitor Karanka commended the performance of goalkeeper Victor Valdes, who made a series of saves in the first half to frustrate City.
Valdes, who won two of his three Champions League titles at Barcelona when Guardiola was their manager, was inspired on his first return to a city he left when he departed Manchester United at the end of last season.
The 34-year-old's performance ensured Middlesbrough extended their unbeaten run to three matches, having drawn at Arsenal two weeks ago before last Saturday's home win over Bournemouth.
"Victor is really important and I am really pleased for him because his first games were really good and as a keeper you need a lot of confidence," Karanka said.
"I tried to transmit my confidence to him and he is showing how good he is. He is a leader in the dressing room, he is really good and he is going to help us a lot.
"Today, Victor made three amazing saves. Against Arsenal and Bournemouth, he made two or three.
"I know when I signed him, he was not the same keeper he was at Barcelona because if he had been at that level, he would not have been here.
"He is with us because he had problems in the past (with injury) but the main thing for me is to keep him happy. He is an amazing keeper."