McClaren lost his job as Newcastle head coach in March with the club in the bottom three of the Premier League, replaced by Rafa Benitez.
McClaren had only been in charge nine months, and believed he would get longer to turn the situation around, citing the backing Alan Pardew received as evidence.
And he says the failure to address the squad deficiencies was a major reason behind his failure – despite the club spending £80million on new players.
Aleksandar Mitrovic, Georginio Wijnaldum, Florian Thauvin and Chancel Mbemba came in during the summer – all targets before McClaren’s appointment.
But the former Toon boss said that the failure to recruit a recognised striker and more defenders was why they struggled last season.
McClaren had pushed for QPR hitman Charlie Austin, but the club baulked at the asking price given the player’s injury record – and as it turned out, Austin struggled for goals and fitness, even after his move to Southampton in January.
Newcastle did spend another £28million in January, but again not on a defender or striker – instead bringing three midfielders, Jonjo Shelvey, Henri Saivet and Andros Townsend, into the club.
McClaren said: “What I didn’t do, and probably what Rafa needs to do, is to take control – and be given control of the whole situation, and especially the football side.
“That is my one regret that I didn’t have that.
“And also that I didn’t really have time – because we said that it would be three windows (to change things).
“We had two, and I don’t think we recruited the right players in the right positions, otherwise we wouldn’t have conceded so many goals.
“We only had one striker – poor Mitrovic on his own for the majority of the season.
“We needed back-up for him, so we didn’t score enough goals.
“We always had to score a lot of goals to win games and, unfortunately for me, I thought with past history (at Newcastle) and with records from Alan Pardew, that I would be given time.
“Unfortunately I wasn’t.”