It was the first time the Stags have failed to score this season and a very disappointing result at a ground where they knew there was every chance of recording their first away win of the season.
Instead Mansfield provided the Robins with only their third win of the campaign thanks to yet another soft goal and McEwan choked: "I just can't believe what happened.
"I never even saw it because I was just coming out of the tunnel and it was in the back of the net.
"I have said before that we do gift silly goals - and there is another one. If we stop doing that then we might have a chance of winning matches. It is very disappointing.
"Their keeper has made some good saves to be fair, a couple of really good ones.
"You have to respect the opposition and if you are not right than anyone can beat anyone - I've said it before and that's proven to be right again.
"There was a lot of effort and commitment from the players but not a lot of thought.
"They gave it a real good go, but I never thought we were in any danger of conceding any other goals other than us committing hari-kari.
"A lack of penetration is a problem we have at the moment. We have know about it, but it is not for a want of trying. We caused them problems with some good build ups but we were disappointing in the final third where we had no end product.
"You can't point to anyone in the team, but there were certain thing we didn't do and they know they have to do better than that.
"Up front we were disappointing but it is pointless blaming individuals.
"I thought we had got a good opportunity of winning tonight and I said to the players before the game it was a good chance for us.
"They had not got a lot of wins under their belt and we were coming off the back of a decent draw against Cambridge.
"We were looking to carry on what was a decent second half performance against Cambridge, but it didn't happen.
"I'm disappointed for the travelling fans that we have not given them more."
He added: "In the first half we played some decent football without carving them open.
"We had one good chance where Mark Stallard headed over and the final ball in could have been a lot better.
"I don't want to give too much away ahead of Saturday, but we know what we have got to work on.
"We miss (Aaron) O'Connor and the way he can stretch teams, but we don't know if he has got a chance for Saturday.
"But we have got two strikers who are experienced and good
players and I'm sure they know they can do better, as do the rest of the team.
"Nothing dropped for us in the final third but we also had problems in the build-up, with the end product.
"Whether it was crosses or people not being aggressive enough with their runs to get in positions, there are one or two players we need more from."
Stags could be without left back Alex Jeannin at the weekend after he limped off with a thigh injury that he been having treatment on prior to the game.
Earlier in the day McEwan had taken the precaution of registering former Sheffield Wednesday youngster James Kay in case of needing another left back though left winger Michael Blackwood eventually did well there for much of the seconds half.
"I thought we lost our shape a bit when Alex when off and Michael had to change position," said McEwan.
"James is just a young boy. Alex Jeannin was not 100 per cent and thought he might be struggling.
"But then he said he would be fit. Michael Blackwood can play there and would have done had he not made it. That is one of the reasons I brought him (Blackwood) in. He can play in several positions and is very versatile,
"James has been training with us for a few weeks and we got him in as a precaution."
McEwan refused to let the press speak to any of the Stags players last night.
After the rosy start to the season Stags have now taken only one point from the last nine and have two investigations into their transfer dealings in motion which could result in points being deducted.
