Scottish Premiership giants Celtic signed defender Simunovic from Dinamo Zagreb in the summer transfer window.
The 21-year-old centre-back has yet to make a huge impact at Celtic Park, largely due to the injuries he has suffered.
Former Celtic midfielder Hay believes that the Hoops should not give up on the Croatia Under-21 international.
“He (Simunovic) has been unfortunate with the injuries that have curtailed how much involvement he has had with the first team but you have to remember that he came with a solid reputation and it is too early to make the call on whether he has been a good buy,” Hay has written in The Glasgow Evening Times.
“The fact he and Dedryck Boyata, who has had a rocky start to his time at the club, have not had the chance to form a decent partnership has led to the chopping and changing that’s left Craig Gordon looking less assured.”
“The fee that Celtic got for Virgil van Dijk was incredible but they did spend a chunk of it on Simunovic and hopefully he can eradicate the injury issues that have curtailed his performances since his arrival,” added the former Celtic manager.
Celtic are at the top of the Scottish Premiership table at the moment with 35 points from 14 matches.
The Bhoys are six points clear of second-place Heart of Midlothian and are set to win the league title in the Scottish top flight yet again.
The Hoops will return to league action on Saturday afternoon when they take on Kilmarnock at Celtic Park.
Celtic will head into the game on the back of a 4-1 victory over Ross County away from home in the Scottish Premiership.
The Glasgow-based outfit will be heavy favourites to win against Kilmarnock, and it will be a surprise if they slip up.