Former Mussels forward Rob O’Toole’s first half penalty was followed by goals from Ryan Warwick and James Fraser after the restart as division-lower Southern Combination League Premier Division Saltdean United sealed a 3-0 second-round triumph at Middle Road.
A late equaliser on Saturday had forced Shoreham to settle for a point in their home Bostik League South clash against Hastings United, and Tuesday’s result brought a frustrating week to a close, according to Mussels manager Donnelly.
He said: “Despite conceding a late goal on Saturday, we played really well and showed what we can do. Everything that was right about that performance was missing against Saltdean.
“I’ve come away from the game very annoyed at our performance. It’s not just a cup game, we had a chance to play in a final at the Amex and the Sussex Senior Cup is a big competition.
“It was an opportunity to build some momentum but we didn’t take advantage of it.”
Shoreham defender Odofin O’Niel was adjudged to have brought down a Saltdean player seven minutes before the break and the referee pointed to the spot.
O’Toole, who played a key part in Shoreham’s SCFL Premier Division title win last term, slammed the spot-kick past James Broadbent.
Mussels fell a further goal behind ten minutes after the restart, before Fraser made it 3-0 20 minutes from time to wrap up a surprise cup win.
Shoreham now face two home matches in the space of three days as they look to climb off the bottom of the Bostik League South table.
Ninth-placed Hythe Town travel to Middle Road on Saturday, then Mussels host two-places higher East Grinstead Town on Tuesday.
Donnelly knows his team will have to be much better than the performance they dished out against Saltdean if they are to take anything from the upcoming matches.
He said: “We were a long way from our best in the Senior Cup defeat, which must be improved on.
“Hythe Town are going to provide us with a tough test on Saturday but we’ve shown at times this season we can be a match for anyone.
“Hopefully we’ll be able to pick up something in that one, then we face a massive game against East Grinstead on Tuesday.
“These two matches provide us with a chance to put some points on the board and an opportunity to move up the table.
“As I said, we’ll need to be a lot better then we were in our cup defeat to Saltdean, where we were not good enough.”