Birmingham midfielder Gardner went in on Di Santo with a foot raised in the final minute of Saturday's woeful Premier League goalless draw at St Andrew's and was given a straight red card by referee Mark Clattenburg.
Blues' four-goal leading scorer will have to serve a three-match ban, but Di Santo could be out for longer if scans show serious damage.
Martinez had a brief chance to assess the damage suffered by Di Santo, and explained it could be a bad blow.
He said: "I've just seen Franco Di Santo's leg and that's my worry. We have to see the extent of that injury.
"He has a couple of nasty cuts in his leg and by that you can see the challenge was really high.
"The referee had a clear view and was on top of the incident and my worry is the extent of the injury.
"The cut swelled up straight away and there could be a bit of ligament damage in his ankle, which is the worry.
"From where I was, Gardner's leading foot was really high and he gets the ball with the second foot.
"Nowadays you can't go into a challenge like that with your feet high.
"It was a reckless challenge with excessive force and it was a red card, but all I'm worried about is Franco's injury."
Birmingham manager Alex McLeish had no complaints with the sending-off.
McLeish said: "At the time I just thought it was two players together and you couldn't avoid a collision but I've seen it on video and we won't be appealing.
"I don't think it was a massively dirty tackle but when your studs are showing, it's come more and more into the English game.
"You might have got away with it a year ago but, when there are studs colliding with a player's leg, then the refs are clamping down on that in this country.
"It's been in international football for a long time but as long as everyone is consistent I don't mind. The ref was in a good position and, under the rules, he's made the right decision."
Martinez praised Charles N'Zogbia for the way he handed abuse from the Birmingham crowd after the collapse of his move to St Andrew's in the summer.
The Wigan boss said: "I thought he coped with it very well. I was never in doubt because he loves that sort of challenge.
"I think in football when you get the opposition crowd targeting you it's a football compliment because they see you as a threat, as someone who can win the game and I felt he took it in the manner.
"He was a threat, worked really hard and is slowly getting back to his best."
Birmingham are now unbeaten in 18 top-flight home games, equalling their best-ever run.
McLeish said: "I am proud of the record. There was a lot of pressure on us today due to the media coverage of the record.
"I felt it could be one of those occasion where people were waiting for you to end up with egg on your face.
"I am sure there were a lot of people predicting we would lose the record just because of all the hype but it was something we couldn't avoid and I told the players that.
"We put it aside and did everything in our powers to get the three points. We just couldn't get the ball into the net.
"There were some great set-piece situations, some great deliveries from Seb Larsson in the first half, but we're lacking that killer touch."