The 20-year-old escaped censure from referee Mark Halsey after a knee-high tackle on the Magpies substitute in the first half of their crucial 2-1 victory.
Newcastle boss Alan Pardew claimed at half-time Halsey apologised to him "if he missed anything" but Martinez is confident his striker will not receive a ban - which would rule him out of their historical Wembley FA Cup semi-final - as a result of television evidence.
"What you need to look at in those incidents is if there is intention, a nastiness about the tackle. We are not a nasty team," he said.
"He (McManaman) has not a nasty bone and is not bad-intentioned.
"The referee was in a perfect position, he is looking at the incident, where the ball is and where the ball is hitting.
"When you get the ball and then there is a follow-up it is very difficult for a panel to punish that."