The 24-year-old required two operations following John McCombe's late tackle that left him in a heap on the ground after he opened the scoring in the Cobblers' 2-0 victory over Port Vale at the weekend.
Boss Aidy Boothroyd said: "We are probably looking at six to nine months for him. He's had a skin graft and his shin is pinned so that is going to be a long job.
"He's very down at the moment, as you can imagine, but he's had lots of good will messages from supporters and he's asked me to pass on his thanks for that.
"I'd also like to thank all those people around the club who acted so quickly on Saturday to get Alex sorted and into hospital, including the club doctor, Stuart Barker the physio and all the St John's ambulance people and the paramedics."
From hospital, Nicholls tweeted: "Just recovering from my 2nd Op. overwhelmed with the calls/msgs/tweets cant thankyou all enough! Feeling v.tired atm will speak 2 u all soon."
- Sports & Recreation