But, the experienced defender, who admitted it felt "strange" to be lining up against players he was on the same side as just 11 weeks ago, said that his new team-mates were caught collectively cold by their Championship opponents.
The Sulphurites' second friendly outing of the summer saw them beaten 3-0, with the first two goals arriving inside the opening 12 minutes as Paul Warne's men took full advantage of what Mattock described as a lack of "bite" and competitive edge.
"It was a little bit strange to be fair, just like Friday [against Sheffield Wednesday] where I ended up playing against two lads who I was playing with last year," he told the Harrogate Advertiser.
"Obviously this game I was up against all the players I was with last year, so it was strange but it was a good game. Rotherham are a good side and it was a little bit of an eye-opener for us first half.
"I think they took us by surprise with how competitive they were and I think that it showed us where we need to be more competitive ourselves. We've got to be open about that and admit to it.
"I think we were caught a little bit cold first half. There wasn't enough bite to us. They were winning every first ball and getting onto every second ball and that's something we have got to be better at because once we started doing it we had the opportunities to bring it down and start playing.
"And, just like the gaffer said to us at half-time, you've got to earn the right to play, and it's true, we should know that from the start."
Rotherham moved ahead when Hakeem Odoffin was able to nod home Dan Barlaser's left-wing corner from close range, then doubled their lead through the latter's neat, low finish from Conor Washington's pull-back.
It would have been hard to argue that the Millers were not good value for that scoreline at the interval, though Alex Pattison - Town's stand-out performer in the first period - came close to levelling matters just seconds after the hosts fell behind, only to be denied by a superb Viktor Johansson save.
The second period was a much different affair, with a Harrogate side featuring no fewer than five trialists giving a far better account of themselves.
And the League Two outfit twice had the ball in the back of the net following composed finishes by Jack Muldoon then Luke Armstrong, only for both strikes to be ruled out due to what looked like dubious offside decisions.
Substitute Ciaran McGuckin would eventually add a third for United on 72 minutes with an effort that Pete Jameson was unlucky not to keep out the back of his net, though Mattock insisted that there were plenty of positives to take from his new team's showing during the second 45.
"Second half, we did well," the 32-year-old former West Bromwich Albion and Leicester City man added.
"We came out and showed a bit more commitment and moved the ball about well. We had two goals disallowed and that showed that there was a little bit more from us second half compared to the first.
"I'm not too sure if the second one was offside. I think Luke was just on, it looked like that from where I was.
"There were definitely some positives. It's only the second game of pre-season and obviously they are two tough games that we have had, which is always going to be good, playing teams from the Championship and League One.
"We've got another tough one on Saturday against Huddersfield, but it's going to do nothing but good for us."
Town's clash with the Terriers kicks-off at Wetherby Road at noon.