Erik ten Hag has denied that Lisandro Martínez aggravated the knee problem that has ruled him out for at least two months by playing on against West Ham, with Manchester United’s manager saying it can be a “grey area” to make the correct decision regarding injuries. Martínez was hurt during the second half of Sunday’s 3-0 win at Old Trafford. After receiving treatment the defender re-entered the contest but broke down again almost straight away and was taken off. The 26-year-old has sustained medial collateral ligament damage to his right knee. Ten Hag was asked whether the problem might have been made worse by Martínez not being substituted immediately. He said: “I don’t think so but it was Lisandro’s decision and he was comfortable to be back on the pitch but after he tried he stopped in the next attack. It is [almost] impossible [to diagnose injuries]. In some situations it is clear and obvious but there is always a grey area and it is very difficult to see from the sideline. “We have the [TV] screens and we see it as well and our medical staff are over there [treating him] but to assess in one or two minutes was impossible. After a game I say: ‘Can’t tell you – you need 24 hours minimum,’ so how can you tell in two minutes?” Alejandro Garnacho scored twice and Rasmus Højlund once against West Ham, with another youngster, Kobbie Mainoo, also impressing. After Garnacho’s first goal, he, Højlund and Mainoo celebrated by sitting alongside each other on an advertising hoarding. Ten Hag was impressed but stated all have to be consistent and deliver against the very best opposition, West Ham having been sixth before kick-off. The trio have started eight matches together and won five of these, the other victories coming against Aston Villa, Wolves, Wigan and Newport. “They enjoy playing together,” Ten Hag said. “There is adventure. They are a danger. They want to do it together and it’s great because that is the key [and] we can have a lot of pleasure from this. There is a lot of space for improvement and that is necessary if you want to go to the top level. This club wants to achieve a lot, to win trophies, and is very ambitious. “So they have to step up and bring higher levels and consistency. Also against the best opposition they have to express the same threat and it is about the end product.” Aaron Wan-Bissaka, who sustained an unspecified injury before the FA Cup win at Newport, had a setback in training on Thursday as United prepare for Sunday’s game at Aston Villa. “Aaron Wan-Bissaka today was training and then dropped out so I don’t know at the moment where he is, the diagnosis; hopefully not too bad,” Ten Hag said. “Mason [Mount] and Tyrell Malacia [are still] not ready.”