United are unbeaten in six games and boss McKinnon has urged his players to take confidence from that run.
The Tannadice outfit, who finished 14 points behind Championship winners Hibs, are looking to make an immediate return to the Scottish Premiership.
"Leading into these play-offs, we are the form side," McKinnon said.
United finished third in the second tier, with their final game of the regular season a 1-1 draw away to Morton on Saturday.
"We should have won the last six games, we have created enough chances to win the six games, which is a real positive for us," McKinnon added.
"We did the same on Saturday. We have hit the woodwork five or six times.
"So it's important to make sure we realise we are in good form. We haven't had the just rewards in these six games but we go into the games in the right frame of mind."
McKinnon's Raith Rovers side lost 2-1 on aggregate to Hibernian in the play-off quarter finals last season.
And he has warned his players they need to perform at their highest standard to progress to face Championship runners-up Falkirk in the semi-final.
"If we're at our best then we'll be difficult to beat," McKinnon said. "So that's the focus going into these games, just making sure we are at our best.
"We went down there on Saturday and played extremely well and we need to do the same again.
"They need to show discipline as well. It's not win or bust, it's about making sure you are in control of your game and in control of your emotions. We have had a chat about that and the players totally grasp that."
Ray McKinnon has told his Dundee United players "there is nothing to fear" ahead of their promotion play-off quarter-final first leg at Morton.