Southend United defender Mark Phillips was taken to hospital after passing out in the dugout as Bristol Rovers struck late to secure all three points.
Phillips was substituted in the second half after a clash of heads with the hosts' striker Matt Harrold.
Harrold's penalty, after two Shrimpers defenders slid in on Mustapha Carayol, secured Rovers' win.
Freddy Eastwood had gone close to scoring for Southend on his return to the club with two headers.
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Southend manager Paul Sturrock told BBC Essex:
"It (the Bristol Rovers penalty) is an unbelievable decision for a referee to make, he was miles away from the incident.
"The linesman hasn't put his flag up, the boys got a shot off into the side netting and he decides it's a coming together.
"Decisions like that can hurt you big time in football.
"It's a killer, I'm absolutely gutted for everybody because I don't think we merited losing that game."