The Colts returned to Lincs League action for the first time since October, and took a while to find their feet.
But after trailing 1-0 at the interval, Nicky Frost and Jamie Elston secured the 2-2 draw.
“Firstly, I will give Immingham credit,” Greswell said.
“They’re not a bottom-of-the-table side and should be nowhere near there.
“First half we couldn’t handle them and, to be fair, they should have killed us off in the first half.
“I don’t know what was wrong with us but we weren’t doing anything right and looked like we had never played with each other before.
“Worst half of football I’ve seen from us in a long time.”
However, Greswell was lifted by the second-half performance.
He added: “To be fair to the boys, I didn’t have to really say anything at half time and they sorted themselves out.
“Second half was much better and we could have even nicked it at the end.
“A draw was a fair result in the end, but it’s a wake-up call for us as the first half performance was unacceptable.”
The poor first half saw Wyberton trailing but the second half was a different story, the Colts come out firing.
They levelled in the 47th minute via Frost’s cross-shot.
Immingham regained the lead from the penalty spot after the ball was adjudged to strike Bailey Forth’s arm.
Wyberton equalised when Frost’s corner found Elston, who headed home.
Both teams had chances to nick it with James Doughty making a smart stop from a free kick and Immingham’s keeper making two good saves to deny Peter Evison and Frost.
On Saturday Wyberton travel to face Horncastle (KO 2pm).