Ajer was linked with a move away from Celtic in the transfer window, and AC Milan were interested in taking him to the San Siro.
The Norwegian was keen on the move, but ultimately AC Milan refused to make the financial commitment that would have forced Celtic into selling the defender.
Ajer will be in action against AC Milan tonight at Celtic Park in the Europa League, and Donati believes his biggest asset as a player is his reliability in different roles in the team.
He insisted that there is nothing out of this world about Ajer, but he is reliable and has the physical attributes of a solid defender.
Donati told Italian outlet Tutto Mercato: “He is a player who plays well wherever you put him.
“A physical player, he doesn’t do anything extraordinary but he is undoubtedly reliable.
“We can compare him to [AC Milan defender Simon] Kjaer, they are similar in terms of their physical stature, sense of positioning and an aerial game.”
Ajer has a contract until 2022 with Celtic and is believed to be keen on a big move away from the club in the future.
AC Milan are still monitoring him and could consider trying to sign him again in January or next summer.