From what I have been able to learn, mobilizing the knee to shape the graft is important in the early stages. The reason why I am protecting it from too much strengthening is that my current aim is to get pain, heat and swelling under control first. I have good ROM (extension just 1/2 degrees off; flexion around 145 to 154 degrees depending on swelling), and overdoing it may set off a reaction that will hinder progress. My knee is still warm to touch and swells.
All the protocols I've looked at have indicated the importance of increasing duration of cycling rather than strength initially. I know that there is some debate about duration v strength in athletic training, but I've elected to follow medical guidance to focus on duration first as the forces on the joint are lower.
CR: you've probably mentioned at some point whether or not your PT is experienced with rehab from MACI, but all the guidance I've seen or can recall emphasize duration initially and only begin strength training from 3 months at the earliest. However, that may be due to the location of my grafts in the PFJ. I think that it's worth while you doing some research online and looking at other people's protocols (e.g. Drs Cole and Minas, see also Chester Knee Clinic site and http://www.cartilage-repair.co.uk/
). There's also Karen Hambly's papers on the Information Hub here as well as her research article that reviews rehab protocols for ACI. This is a bit dated though since it was published in 2005 and technology has moved on since then.
I remember someone posting that they just self-rehabbed for one year doing nothing but mobilization, quad sets, SLRs, bike at zero resistance and then went to see a physio. Apparently his OS (Dr Gomoll I think), said that it was a good idea because too many physios push you too hard in the early stages. Karen Hambly's paper sets out the earliest time at which you can introduce certain changes to your routine so that's probably a good indicator of where you should be.
All the best!