Pittman joined the action in Saturday's League Two clash against Northampton at the Kassam Stadium as a second-half substitute and fired in the winner 11 minutes from time, driving into the net from 12 yards from Peter Leven's pass.
That was just three minutes after the in-form Cobblers had equalised through their own super-sub Clive Platt.
Lewis said: "JP's come on and looked really lively. He has that ability to change a game. We need to get a bit more out of him, but he was terrific."
Oxford, languishing in 18th place before the game, turned in one of their best performances in weeks, and thoroughly deserved their victory over a Cobblers side who came into the game on the back of a three-game winning streak in the league. James Constable fired the U's in front after 14 minutes from Alfie Potter's low right-wing cross.
And although Constable looked offside when he put the ball in the net, Lewis felt Oxford deserved some luck.
He added: "We hit the woodwork twice right at the end on Tuesday at Barnet and we hit the woodwork twice again here, so I think we deserved a bit of fortune.
"That's if it was offside. And I don't want to sound like Arsene Wenger, but I honestly didn't see it properly, so I can't really judge."