United, who lost 2-1, were told by the Scottish Government and the Scottish Football Association's medical department that the player was clear to play after subsequently testing negative in an NHS test.

Micky Mellon's side were unable to train on Tuesday following the positive result but they got clearance later that day in what is a departure from earlier rules.

Hibernian midfielder Alex Gogic missed a defeat by Aberdeen and his first Cyprus call-up after recording a false positive earlier in the season and Hamilton saw three players enter self-isolation for 10 days following false positives.

The apparent rule change was news to Motherwell and also to United until they were given the all-clear to field the unnamed player.

Alexander said: "The feeling around the club is we were unaware of any rule changes but I didn't want to get involved in that because I wanted to focus on the game.

"It's not for me to comment on another club's players but hopefully it's been handled the right way.

"If there is a change and we haven't had the communication then that's not right. But I don't know if that's the case."

United manager Mellon said: "We did what we needed to do, we spoke to who we needed to speak to, and we waited for the people to come back to us with the necessary information."

Mellon was more concerned about a third defeat in a row which saw Motherwell move three points behind United.

The hosts dominated in the first half and Devante Cole and Christopher Long netted inside five minutes of each other. The second half was much more competitive and United came close after Ryan Edwards headed home in the 81st minute.

"Disappointed that we didn't answer the questions when the game changed with a couple of free-kicks," said Mellon, who felt they were denied "real stonewallers" following several penalty claims early in the second half.

"Sometimes you get asked to roll your sleeves up and dig in and deal with things and we didn't do that enough after the first 20 minutes.

"The thing that makes it more confusing and probably makes you a wee bit angry is that they find another 30 per cent in the second half and look like a completely different team. We have to look at ourselves and ask why we can find that in the second half and don't start with that level of intensity."

Alexander was delighted with his front three of Long, Cole and Tony Watt, who all had several chances.

"I think that's what won us the game in that spell when we were on top – the front three really caused them problems," he said.

"And then for the rest of the game, the rest of the team did what they had to do to keep it down to one goal."