Question: Why did Cao Cao's generals prove unworthy for war at the Battle of Red Cliffs according to Zhou Yu?
About the article "Battle of Red Cliffs", section "Analysis".

Candidate passages:
In Luo Guanzhong's historical novel "Romance of the Three Kingdoms", Cheng Yu offered a ruse in order to get Xu Shu to serve Cao Cao.
At the time Xu Shu was serving as rival Liu Bei's key strategist and managed to score a major victory against Cao's generals Lü Kuang (呂嚝), Lü Xiang (呂翔), and Cao Ren (曹仁).
Exploiting the fact that Xu Shu was an extremely filial person, Cheng Yu suggested to Cao Cao to hold her mother hostage and force Xu Shu to leave Liu Bei and serve Cao.
Cheng Yu wrote a fake letter to Xu Shu and successfully tricked Xu to come to Xuchang.
Ironically, Xu Shu's mother committed suicide after seeing her son fall for such a ruse and leaving a righteous person like Liu Bei to serve under the ruthless Cao Cao.
Prior to the Battle of Red Cliffs, Cheng Yu had predicted that Sun Quan's forces would use fire to attack Cao Cao's naval fleet.
However, Cao Cao did not heed his advice seriously as the winds were to their advantage then.
After Cao Cao's major defeat, Cheng Yu was one of the few advisors who stood by Cao Cao all the way during their escape. 
Zhou Yu's death is mentioned in Chapter 57 of "Sanguo Yanyi".
In the previous chapter, Zhou Yu came up with a plan to help Sun Quan seize Jing Province from Liu Bei.
He pretended that he wanted to help Liu Bei attack Yi Province (covering present-day Sichuan and Chongqing) in western China, and asked for passage through Jing Province.
When Liu Bei agreed, Zhou Yu was overjoyed because his true intention was to conquer Jing Province when his army passed through.
However, Zhuge Liang saw through Zhou Yu's ruse and set up a trap in Jing Province.
Zhou Yu fell into the trap and was surrounded by Liu Bei's forces.
He was overwhelmed by fury and fell off his horse.
Zhuge Liang later wrote a letter to Zhou Yu, asking the latter to give up on attacking Yi Province and return to Wu instead because Cao Cao might take advantage of Zhou's absence to invade Wu.
Zhou Yu then wrote to Sun Quan and told the other Wu generals to give their full support to their lord.
He fainted afterwards, regained consciousness and sighed, "If (Zhou) Yu were to be born, why must (Zhuge) Liang exist as well?"
He died after repeating that line several times.
This was the third time Zhou Yu had been infuriated by Zhuge Liang after sustaining an arrow wound at the Battle of Jiangling, a follow-up to the Battle of Red Cliffs.
The first time was when Liu Bei, acting on the advice of Zhuge Liang, seized control of many territories in Jing Province when Zhou Yu and his men were busy fighting with Cao Cao's forces for control of the province.
The second time was when Zhuge Liang foiled his "beauty scheme".
(See #Liu Bei and Lady Sun's marriage.)
Zhou Yu's condition worsened after each incident and he eventually died on the third time. 
Sun Ce was assassinated in 200 CE while he was out on a hunting expedition and was succeeded by Sun Quan.
In 208, Zhou Tai participated in the Battle of Jiangxia against Huang Zu.
Later that year, Zhou Tai fought at the Battle of Red Cliffs under the command of Zhou Yu and Cheng Pu, and the allied forces of Sun Quan and Liu Bei scored a major victory over Cao Cao's forces.
Zhou Tai was also involved in the Battle of Jiangling, a follow-up to Red Cliffs, and Sun Quan's forces defeated Cao Cao's general Cao Ren and captured Nan commandery (南郡).
After the battles, Zhou Tai was ordered to garrison at Cen (岑). 
Sun Quan put Zhou Yu in command of his 30,000 troops, largely stationed on naval vessels.
Zhou Yu set up a defensive position in conjunction with Liu Bei, whose army garrisoned on land.
Around this time, a spreading plague significantly weakened Cao Cao's forces.
Zhou Yu's subordinate Huang Gai pretended to defect to Cao Cao's side and was accepted by the latter.
Huang Gai brought with him a small group of men and they sailed towards Cao Cao's base on boats.
As the boats approached Cao Cao's fleet, Huang Gai ordered his men to set the boats on fire, and the burning boats crashed into Cao Cao's larger ships.
The fire spread out of control and completely destroyed Cao Cao's entire naval fleet.
Cao Cao's land forces at Wulin (烏林; in present-day Honghu, Hubei) were also attacked and driven back by Sun Quan and Liu Bei's armies.
Cao Cao suffered a drastic defeat at the Battle of Red Cliffs and was forced to retreat north back to Jiangling (江陵, located in present-day Jingjiang 荆江, not to be confused with present-day Jiangling County, Hubei). 
Guan Yu was sent to guard Huarong Trail (believed to be near Dongshan Town, Huarong County, Hunan) and intercept Cao Cao when the latter was retreating after his defeat at the Battle of Red Cliffs.
Initially, Zhuge Liang was reluctant in allowing Guan Yu to take up that responsibility, since he was worried that Guan might take into account the kindness Cao Cao showed towards him when he temporarily served Cao years ago, and release Cao as a result.
Guan Yu insisted, claiming that he had already repaid Cao Cao's kindness by slaying Yuan Shao's generals Yan Liang and Wen Chou and lifting the siege on Boma.
He made a military pledge not to let Cao Cao off, while Zhuge Liang also stated that he would give up his head to Guan Yu if Cao Cao did not pass through Huarong Trail.
As Zhuge Liang predicted, Cao Cao did reach Huarong Trail and encountered Guan Yu there.
However, Guan Yu decided to spare Cao Cao in consideration of their past dealings, and after taking pity on the plight of Cao Cao's men and Zhang Liao, whom he saved from death earlier after the Battle of Xiapi.
He allowed Cao Cao and his men to pass through Huarong Trail unharmed.
When he returned to camp he confessed the truth to Liu Bei and Zhuge Liang.
Zhuge Liang ordered Guan Yu to be executed for failing in his mission and agreed to pardon Guan when Liu Bei intervened.