The Swedish full-back appeared to take hold of O'Shea's shirt as the United defender attempted to reach Ronaldo's deep cross.
Referee Peter Walton and his assistant agreed on the penalty and Ruud Van Nistelrooy made no mistake from the spot for what turned out to be the winner after 41 minutes.
"It's always give and take in the box but I'm involved, it's my fault and what more can I say?" conceded a disappointed Erik afterwards.
"I haven't seen the replays yet and these things happen 50 times in a game. Still, it's happened, it's my fault and I'm responsible. That's the way it is."
Clear-cut chances were at a premium especially in that first half and United continued to keep us at distance in the second half although Roy Carroll saved well from Simon Davies and Robbie Keane.
At the other end, Paul Robinson maintained his excellent form with saves from Ronaldo and David Bellion.
"I thought we played quite well in the second half," added Erik. "We fought well and tried to get the equaliser.
"All we can do now is go back to the training ground and work hard for Everton next week."
Erik Edman wasn't convinced his tussle with John O'Shea warranted a penalty in defeat against United but bravely admitted: "It's my fault."