Tanswell says it was PCL injury. Without knowing the extent of the damage or indeed anything meaningful about knee injuries, a brief Google suggests potentially even longer rehab than an ACL. Keeping everything crossed, poor guy has had such awful luck.
This is sort of true but you're missing the most important part. A severe tear to the ACL is, relatively, fairly easy to do, whereas getting that level of injury to a PCL is really difficult and, if it happens, is normally accompanied by other injuries.
The really nasty injury I got which had me out of sport for nearly 2 years was a grade 3 PCL injury but also had damage to my ACL, Medial and Meniscus. Given it sounds like this is purely a PCL injury but it needed surgery I'd be very surprised if it isn't a grade 2 tear where the surgery is fairly minor (I'd doubt there's a graft of any sort for example). If it's what I expect I'd guess he'll be back on grass around the end of the season and should be able to join full training for pre-season. This one is slightly difficult to judge though because surgery on this sort of damage is very rare, outside of sports they'd pretty much never even give it as an option, instead just focusing on natural recovery which would be anything from 3-9months, that we've gone ahead with surgery points to the latter end of that scale with us planning on the timeline I suggest of back for pre-season.
If it was more serious and he'd had to have full reconstructive surgery then yes 12-18months before he's playing again wouldn't be a surprise but nothing that has been said is suggesting that sort of timeframe.
