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.
Malwarebytes can remove Trojan.Injector without user interaction. After the scan and removal, a reboot is required. Malwarebytes will prompt you to do this.