Defender Gathern was walked off the field immediately after a clash of heads early in the second half of Mussels’ 4-2 Bostik League South loss.
Donnelly revealed Gathern looked ‘disorientated’ having been hauled off and was taken to East Surrey Hospital.
The Mussels defender remained in overnight and was due to go for tests this morning.
Donnelly said: “My main worry at this moment is the health of Alex (Gathern).
“He had a nasty clash of heads with a South Park player and was really disorientated after coming off.
“We got someone to take him to East Surrey Hospital, where he remained overnight.
“At times like this football does not really matter – my main concern is that Alex is fine and well.
“We really struggled after he was taken off but, as I said, the main worry is making sure makes a speedy recovery.”
South Park struck three times in 15 second-half minutes to run out winners in a open encounter against Mussels.
Jack Mazzone made it 1-0 on 19 minutes but back came Shoreham, levelling through frontman George Gaskin three minutes later.
Mussels were still in at the break, before the home side netted three times in quick succession. Chris Smith fired South Park ahead for a second time eight minutes after the restart.
James McEllington made it three 23 minutes from time, then Mazzone’s spot-kick made it 4-1 less than 60 seconds later.
Shoreham stuck at it, getting another through Ross Myers after 72 minutes but the home side stood firm.
Defeat for Mussels stretched their winless league run to 16 – with 12 of those losses.
Donnelly’s side find themselves 14 points adrift of place-above Molesey and safety but improving signs in recent weeks have given their boss hope.
“We’ve not actually been playing that badly, it’s just about getting a break and stringing some results together,” Donnelly added.
“The game (at South Park) was so open, it could easily have ended up 5-5.
“The challenge and task facing us is trying to string some results together as the season goes on now.
“We’ll continue to work hard, put in the effort we have been over the past few games and hopefully things start to turn around.”
Following back-to-back matches on the road, Mussels return to Middle Road and take on Phoenix Sports on Saturday.
Donnelly is targeting a first win in 17 league matches in that one.
He added: “We’ve been waiting so long for a win, let’s hope we can finally end the wait on Saturday.
“Performances have been much better, it’s only a matter of time before we pick up three points.
SHOREHAM: Broadbent; Koduak, Packer, Gathern, Storrie; Elgar; Lyle, Amdzade, Myers, Gritt; Gaskin. Subs: Bullivant (Gathern), Collyer (Koduak), Whitemore, Gates.