Coming out of their mid-season bye, Gunston trained on Monday and the Hawks were hopeful he would come up for Friday's away clash.
But he did not pull up well and sat out Wednesday morning's session.
Gunston has only played nine games this season. His most recent match was in late May against St Kilda, when he kicked five goals and was rested for the final quarter.
"We just can't quite get that mid-foot sprain as sorted as we'd like," coach Sam Mitchell said before training.
"Unfortunately he just hasn't pulled up as well as we would like, so we just need to continue to re-assess."
The good news for Hawthorn is that defender Tom Barrass is set to return. He has been out of action since round 10 with a hamstring injury.
In their most recent match, the Hawks only managed one goal after halftime and were overrun by the Western Bulldogs.
Mitchell is not thinking about wholesale changes for Friday's match.
"It's not throwing the team up and starting again," he said.
"We had 15 shots ... for one goal in the second half. There's a part of me that thinks our system is not right - if we're scoring that few goals, then the system must be wrong.
"But if we're getting that many shots, if we just fix a couple of things, we're not too far away from putting together a better performance.
"Some of our best footy of the season was played in that game - we need to get the balance right."
While the heat is on the Suns amid a three-game losing streak, the Hawks also have problems. They only have two wins and a draw from their past six matches.
"To be where we want to be, come the later part of the season, we have to beat teams like the Suns. This is a massive game for us," Mitchell said..
"We'll see how quickly we can bounce out of what has not been our best form."
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