Evans was sent off 15 minutes from the end of Manchester United's 1-0 win over Bolton after Holden sustained a nasty gash in a tackle with the Northern Ireland international.
Coyle claimed at first sight he felt it was just a full-blooded challenge between the two players.
But Holden came off worst and as Bolton players called for immediate assistance, referee Andre Marriner decided the tackle warranted Evans' expulsion.
"Stuart has gone to hospital," confirmed Coyle.
"I think the players reacted the way they did because a cut opened up on his knee. We hope it is nothing worse than that and there is no break or ligament damage but we will only know when we get the results back from the hospital."
Source: PA
Source: PA