Trojan.Agent refers to a generic Trojan malware detection. It is used for detections that are either associated with no specific malware families or not enough information is available to pinpoint the malware family.
Trojan.Injector is the generic detection for malware that injects itself into other processes or files. This is a very effective method to hide from the average user as they will only see the regular active processes.
Malwarebytes protects users from Trojan.Injector by using real-time protection.
Malwarebytes can detect and remove Trojan.Injector without further user interaction.