During liner replacement on an F-1000 mud pump, a liner pusher tool was clamped to the pony rod to push the liner forward for removal. As the liner was still partially inside the fluid end, an extension tool was added between the pusher plate and the liner. The extension was held at the base of its handle; when the pony rod was manually cranked forward, the extension slid down and misaligned, and the momentum caused IP’s left index finger to be pinched between the extension tool & the pusher plate. The injured person was treated at MAC 4 & later transferred to Shaleem Hospital for an X-ray