Former Liverpool right-back Glen Johnson believes the club have to sign an upgrade on Konstantinos Tsimikas this summer. The Greece international started the Premier League game against Chelsea on Sunday (May 4), as Andrew Robertson was rested.However, Tsimikas struggled to impress, as the Blues ran riot at Stamford Bridge, picking up a 3-1 win. With Robertson already 31, the Reds may have to lay down succession plans for the position soon.The Scottish left-back was indispensable for the Merseyside club in recent years and has been a key ingredient in their success. However, Robertson has displayed signs of regression this season, registering just one assist from 43 games across competitions.Tsimikas was expected to eventually replace the ageing left-back, but has hardly fared any better. The 28-year-old has managed two assists from 27 appearances in all competitions this campaign.Speaking to Premier League Production after the Chelsea game, as cited by The Metro, Johnson warned that Liverpool cannot rest on their laurels after winning the league."Today, though, I thought Tsimikas really struggled – it wasn’t his best game. There were a lot of Liverpool players that were under par by their standards, but I definitely think they need to sort out the left-back side," Johnson said.Johnson continued:"You always have to stay in front of the game. They are champions and the best team in the league, but you can’t dwell on that. They need to get some players in to improve them next season because you can’t rest on your laurels."Robertson is under contract at Anfield until 2026.Do Liverpool have a ready replacement in their squad for Trent Alexander-Arnold?Trent Alexander-ArnoldTrent Alexander-Arnold is expected to depart Liverpool at the end of this season, once his contract expires. The English right-back hasn't signed a new deal yet and remains heavily linked with a Bosman move to Real Madrid.Johnson, however, believes that the position won't be a concern for the Reds because of the emergence of Conor Bradley."The door looks open for Trent [Alexander-Arnold] to depart but that’s less of a problem with (Conor) Bradley coming through. Even today, he came on, looked sharp and he was a threat – he almost created two chances in the first few minutes of coming on. So he could be a massive player for Liverpool," Johnson said in the same interview.Alexander-Arnold has registered 23 goals and 92 assists from 351 games for the Merseyside club to date.