Signing Keane as a fourth choice center back is a no brainer. We have three center backs ahead of him, and there isn't a single free agent who is 1. better than him, 2. liable to be happy with the role available, and 3. not a big injury risk. To get anyone better, you'll have to pay a transfer fee of at least a few million.
Again, we have Branthwaite and O'Brien with Tarkowski probably returning early in the season, and the just team promoted the extension of Eli Campbell with a quote about looking to get into the first team. If Campbell has a good summer, he can become our Jake O'Brien on the left, able to cover for both Mykolenko and Branthwaite.
If we can sign Keane on a reduced wage, it's cheap security. If we want something better, then we're better off spending money for someone who can compete for a spot now or in the future.