The rule - which was brought into effect when the Scottish Premier League and Scottish Football League merged last summer and was not highlighted to clubs - means that a club can now only send one player out to each side in the league system.
Although Motherwell managed to complete the signing of Estonian striker Anier last night, McCall was furious that much of the club's work over the past week could have been for nothing. The Fir Park club had freed up money to offer erstwhile loanee Anier - his short-term spell from Viking Stavanger was to expire after today's match at Hearts - a permanent deal and had balanced the budget by arranging to send a second player to Dundee, to join Adam Cummins on loan.
That loan arrangement, as well as the deal for Anier, was about to be rubber-stamped when McCall was informed by an agent that it would not be possible.
"I'm flabbergasted, annoyed and stunned," admitted McCall, before Anier was signed belatedly. "Dundee didn't know. They came to me, and said 'can we take the two of them?' I didn't even think about it. We're trying to develop our young boys, bring them through and help them. You know what it's like with young boys, you want to go with your pal if you want, share the digs, the expenses, whatever it is.
"I got a text from an agent last night, to flag it up, because he'd heard of another manager trying it in the lower levels not long ago. Now, why it wasn't flagged up to anyone then?"
Last season's SPL rulebook stated that only one player could be loaned out to each top-flight side, a law put in place to stop richer teams bolstering their rivals with a group of players who would not be able to play against their parent club, therefore giving them an advantage. SFL rules, which were designed to span three divisions, were less stringent.
But rather than create an adaption of the two variations,
the new body chose instead to continue with the SPL version, which has no allowance for more than one player moving between leagues to the same club.
"This rule of [loaning only]
one player to a club, certainly when you go lower down the leagues, it's insane, it's crazy," said McCall. "We didn't even give it a thought, because you look
at Dunfermline, having three Dundee United players, you look at St Mirren having two from Newcastle. You wouldn't even think there was a change."
It was reported on Thursday that Dunfermline will escape punishment for breaching the rule, as they were unaware of the change to regulations - and were not informed when registering the players - when those loan deals were organised. St Mirren's later acquisition of two Newcastle players, on the other hand,
is apparently legal since cross-country arrangements are tackled under FIFA regulations, rather than the SPFL.
"When you go to referees' meetings at the beginning of the season, we don't go through every rule, but we go through rules that have been changed," said McCall. "So why hasn't this been flagged up? And why can Newcastle send two to St Mirren? It's barking mad."
McCall and Motherwell
will raise the issue at an SPFL meeting at Hampden on Monday, and while SPFL officials are believed to recognise that the rule should be changed, it is unlikely that any amendment will be made in time to conduct business during this transfer window.
Anier, meanwhile, was delighted to have eventually signed on at Fir Park. "I am really happy things are tied up and my long-term future is sorted out," said the striker.