Campbell has Portsmouth fears after January player sales
He said: "If you don't replace players, then it's not rocket science. If any side loses four good players they're not going to be firing on all cylinders.
"We were cruising until we lost all those players. We only needed another two or three really good quality players and then we would have been in Villa's position - no question. We had a fantastic team.
"There was potential there at Portsmouth, but I really do think it's gone now.
"Getting back to the level of European football on a regular basis is going to take a big effort over the seasons. But at least we have turned the corner under Paul Hart."