Oxford boss Chris Wilder labelled Caolan Lavery's opener as 'poor' after they lost 3-2 at home to Plymouth.
The Sheffield Wednesday loanee got things going with a first-time drive from the edge of the box and it put the Pilgrims on the front foot.
Asa Hall levelled for the U's with a 25-yard shot, but Reuben Reid struck twice in the last nine minutes, one of them a penalty after Dave Kitson handled, to give Sheridan's team all three points.
Deane Smalley grabbed a late consolation but it proved to be just that.
Wilder said: "I wasn't too unhappy with the performance, but I was unhappy with the goals we conceded.
"For the team with the best defensive record in the league to concede three goals like that was very disappointing.
"We gave away a poor goal at the very start of the second half.
"We got a great goal to get it back to 1-1 and with half an hour to go I really thought we would go on to win the game.
"The penalty changed it, but I've got no complaints. It was a penalty. And for their third goal, we were massively chasing, and that happens.
"For us it's a missed opportunity but we'd put a decent little run together.
"And if we replicate what we've done in the first half of the season in the second half, we won't be far away.
"We would have liked to have opened up a bit of a gap between the top three and the pack below, but none of the top three won, so it was a good day for the chasing pack. But the good thing is that we remain in second place."