A single endorsement is not going to change the race.
Assuming it stays as a single endorsement, it probably won't. But there are already rumors of a Giuliani endorsement, and other Republicans know that coming out early (assuming they believe in a Trump victory) will pay higher dividends than sitting on the fence, especially with someone like Trump who takes these things personally.
As I said, I predict that this will be a Trump - Kasich race from March 15th (I am on record regarding this prediction, here on Atlas, so we will see if I am smoking something "really good", or if I am clairvoyant)!