"There's a lot of scar tissue which means it has healed really strong but it also needed another operation to clear some of that up.
"So it's just taken more time and I couldn't train very much and I lost some strength in my leg."
Cooper got back on his Honda CRF450 about six weeks ago "for some play riding, not hard motos" and rode at Pukekohe earlier this month at the Auckland Champs.
He's made solid progress in recent weeks.
"I've set myself a level to ride at for Summercross which is hopefully good enough to win again. If it isn't, I won't be pushing over that level at this early stage."
Cooper remains with the Honda Shop Racing squad for the coming season.
"We've been testing the 2015 Honda and it's only little changes over last year's bike which I really liked."
Carter had made the move to the MX1 class for the Australian Nationals and put together a solid run of top-10 results for the Suzuki team until he broke his wrist at a Taupo event in July.
"Everything was going well in Australia then a little hiccup comes along and I had to miss the last three rounds.
"It was a pretty serious break. I broke it in four places but I saw my surgeon the other day and he said it's come together beautifully."
Logan Blackburn (Te Puke) is further along the comeback trail after a broken collar bone at the end of last season and an off-season ankle reconstruction.
Previously a title contender on the 125cc bikes, Blackburn is now riding in the MX2 class with the Suzuki team.
A more recent injury will keep Tauranga's Aaron Wiltshier off the track for a few more weeks.
Wiltshier, 18, broke his right foot at the Taupo Labour weekend event. He had the cast removed last week and plans a return to MX2 racing for the Honda team at the Woodville NZ Grand Prix at the end of January.
Racing is for the mini and junior classes on Saturday with the senior - including veterans and womens - titles raced on Sunday.
