Yeo was sent off half an hour into the game at Sincil Bank, and the Imps found it hard to get back into the game having already conceded just six minutes in.
Alexander said: "It was always going to be difficult once we were a man down, I was very disappointed with the sending off.
"I am not making excuses but I thought we deserved better than that against one of the better teams in the division."
Oxford manager Ian Atkins however felt that his side did enough to earn the three points.
He said: "We were always in control but we were a bit too elaborate at times.
"It was just disappointing that we didn't manage to get a second goal but Lincoln is never an easy place to come to."