Danny Cowley’s side are 12th in League One, seven places below the Millers, after one win in their last 10 matches in all competitions - even if that was 4-0 against Sunderland.
Millers manager Warne says it masks the reality.
“They’ve changed their system, they’ve gone to a back three,” he said. “They have struggled with results but not really performances. They lost at Burton, but they were excellent.
“We’ve tried to watch as many games as we can and they’re playing really well.
“Performance-wise I think Danny’s really pleased with what they’re doing because I’m trying to read as much press as I can.
“His Lincoln side were very organised, they do press and get out quickly when they’re defending corners, they are very good at set-pieces themselves and very hard to break down.”
One thing Portsmouth and some of League One’s other big hitters are up against is their manager has only had one transfer window whereas the top five all have greater stability, typified by Warne, fast approaching his fifth anniversary as Rotherham manager.
“Some of these other clubs have still got unbelievable talents so when they do get the magic formula, they could be a threat but perhaps they have such a plethora of options it’s hard to get the perfect fit,” argued Warne.
“You get stability through results and you get the dressing room’s trust if you win. If you’re winning they’ll walk through fire for you, if you lose a few you get people going, ‘Does this man know what he’s talking about?’
“I’ve just tried to keep my best players here as long as I can and when I lose them replace them with similar recruits.
“We’ve probably been a bit braver than in previous League One campaigns which might come back to bite us but we think there’s more emphasis on us. In previous years we probably didn’t have as much belief in ourselves as we should have so we’re probably not as hard on ourselves.”