Very sorry to hear about the injury.
Tramadol can be of questionable benefit in many pets because it is not very bioavailable. Meaning, the vets really have to increase the dose because the pet is not metabolizing it correctly to get benefit for pain management.
This is why many vets are now switching over to Gabapentin for pain relief instead.
In addition, with any type of joint or knee injury, most vets will also do some type of a non-steroidal anti-inflammatory like Rimadyl, Previcox, Metacam, or Deramaxx. That is very important part of pain management.
Yes, you could give benadryl but might not have desired effect. The dose is typically 1 mg per pound, but I have a hard time recommending you give 4 TABLETS OF BENADRYL! at 25mg each.
So, i would say start with just 2 or 3 tablets of benadryl.
I would recommend talking with your vet about trying a NSAID like Rimadyl, and switching pain medications or adding tothe Tramamdol with a drug like Gabapentin.
Hope that information helps