The 64-year-old has been forced to contend with speculation surrounding his future for most of the second half of the Premier League season.
Van Gaal was finally put out of his misery on Monday night after United terminated his contract just days after winning the FA Cup.
The Dutchman became the first manager to win some silverware since Sir Alex Ferguson’s retirement after a 2-1 extra-time victory over Crystal Palace at Wembley last weekend.
Former Liverpool striker Collymore wasn’t impressed with United’s handling of Van Gaal’s departure amid reports Jose Mourinho will take over the reins at Old Trafford.
“So it’s done and dusted then, the dirty secret that we all knew but LvG didn’t,” Collymore told Boyle Sport.
“I think it’s borderline shameful that people were briefing journalists after the Cup Final, not even allowing a decent, if prickly man to enjoy his moment, but we all knew it was happening, so what next?”
Van Gaal was named as David Moyes’s successor in May 2014.
The Red Devils finished in fifth place in the Premier League table behind Manchester City on goal difference.
The 20-time English champions will be without Champions League football for the second time in three seasons.
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