The 19-year-old scored as England's Under-21 side beat Norway 1-0 on Monday evening.
"He made that move as so many young players do and they end up sitting on the bench, they stagnate," Mills told BBC Radio Suffolk.
"He needs to kick-start his career, he really does.
"He should have stayed at Ipswich - if he could have done. I don't know if there was pressure for him to go when that type of money was offered, but at that age you need games."
Last month, Ipswich boss Paul Jewell said he believed the club had first call if he was made available, but they could not afford him.
"I also think we've got to pay three quarters of his wages and we haven't got that type of money," he said.
Mills was delighted to see Wickham make an impact for the national Under-21 side.
"It's terrific that he had the opportunity to play some football - he got the winning goal and that should give him a great boost.
"I do anticipate that he probably will go out on loan and whoever gets him will get a very hungry youngster," he added.