The Sweden centre-back blamed a lack of strength in depth for the club's indifferent form in the second half of the season.
He said: "Looking at the teams we had ahead of us, it is not reasonable to compete with them over an entire season. Among those that we see as our competitors, we finished first.
"Autumn was amazing. We were in the top three or four at Christmas. Then there was the Achilles heel of our team that the squad is not the biggest.
"It tends to show itself in the spring when there are worn-out bodies. It has been a common thread of my time in West Bromwich."