Eastwood was back in the first team after Oldham's decision to recall Alex Cisak early from his loan spell at Fratton Park.
Whittingham on Friday signed experienced former Swindon goalkeeper Phil Smith on a one-month deal, but gave 23-year-old Eastwood the nod against the leaders as Pompey secured a battling 2-2 draw.
He said: "Simon made a great save at the end. People can lose concentration towards the end of a game, but he kept his very well.
"He kept us in the game in the last few minutes and allowed us to go up the other pitch and get a goal ourselves."
Whittingham has backed Smith, 32, to provide some quality competition in the goalkeeping department.
"He's looked good considering he hasn't done much since the summer," he said.
"We've brought him in because of the vast amount of experience he can offer us. He's also got a great work ethic and has a desire to do well."