Lendl helped Murray win two Grand Slam titles, but it was announced yesterday that the two are parting ways after just over two years together.
Andrew Castle told STV he was "absolutely amazed" by the unexpected news and did not see it as a positive development.
"I was with Ivan Lendl two-and-a-half weeks ago and sat with him for hours," said the former British No. 1.
“He was in very high spirits about working with Andy and all aspects. For me it feels a bit negative that Lendl and Murray have split."
Meanwhile, former British player Barry Cowan told Sky Sports that he was "very shocked and surprised," but that it could "benefit" Murray.
"Now I think they have just reached an amicable stage where Andy needs to work it out for himself, what he wants for himself for the next two, three or four years," he said.
"Andy is a special talent and a great player, but clearly if you look at his performances this year he hasn't looked like someone who is necessarily enjoying his tennis on the court.
"He also had that back injury which clearly affects your mentality but what is important, and we haven't really seen this from Andy since he won Wimbledon, is what are his next goals."
Murray won Wimbledon last year, but underwent back surgery after the U.S. Open and has not reached a final since returning at the beginning of this season. Though he says his play is improving, the world No. 6 struggled to explain his lack of "confidence" in a three-set loss to Milos Raonic at Indian Wells.
But former No. 4 Greg Rusedski thinks Lendl will be difficult to replace. "Given that the average coaching relationship on the ATP tour lasts about 18 months, you could argue that Ivan Lendl's stint with Andy Murray -- which extended just a couple of months over two years -- did better than expected. However, the odd thing is that they should abandon a partnership that has been so blazingly successful," he said in a column for the Telegraph.
"Maybe Ivan felt he contributed everything he had to their partnership. [But] without him, it will be that bit harder for Andy to life his momentum and intensity in the build-up to his Wimbledon title defence."