just a note, i myself had a complete acl tear, this was confirmed through arthroscopy and repaired, without having a large haemarthrosis/effusion. i never had more than a minor effusion.... until I had the surgery! That was a different story! an Wikipedia reference-linkmri showed just "acl disruption". haemarthrosis is present in the vast majority of complete tears but not in 100% of same.

If a client of mine were reporting repeated instances of instability and objectively there was laxity on lachmans/anterior drawer I would be looking to see if an MRI was possible. especially if the client was very active etc.