Ronald Koeman has warned Everton's players their former colleague Romelu Lukaku's purple patch will continue if they do not improve defensively at Old Trafford.Belgium forward Lukaku left Goodison Park for Manchester United this summer in a Â£75million deal after scoring 20-plus goals in each of his previous three seasons with the Toffees. The 24-year-old already has six in as many games for his new club and though Koeman knows his defenders will be fully aware of the threat Lukaku carries, he stressed they must tighten up at the back or they will be undone by a familiar face."We know him really well," Koeman said of Lukaku. "He's one of the best strikers. "I mentioned several times last season he's a great finisher. I hope that we know how we have to defend against him. "There are a lot of individual qualities in the players around him (at United). In the box he's really dangerous, he's fast, he's strong."If we defend how we've defended in the last few games then maybe it will be his afternoon."In a busy summer where Koeman overhauled his squad, the transfer window still closed without the Dutchman landing a replacement for Lukaku, who scored 25 of their 62 Premier League goals last term.Ex-United forward Wayne Rooney is the only player to have found the net for Everton in their opening four top-flight fixtures and Koeman accepts that current strikers Dominic Calvert-Lewin and Sandro Ramirez, 20 and 22 respectively, cannot be asked to assume Lukaku's role."There's no frustration now because you can't do anything at this time," Koeman said of failing to add another attacker."It is different in our ball possession if you have a target man in front. You saw with Atalanta (on Thursday), their striker (Andrea Petagna) was strong, he kept the ball, it allows the team to come up and keep the ball."We have different types in Dominic and Sandro. Sandro needs time to adapt to the Premier League. Yes, he has pace but he's not physically the strongest."But it's too easy to mention the strikers. Defensively we are making too many mistakes."Lukaku's new home has actually been a happy hunting ground for Koeman in the past.His Southampton team claimed victories over Louis van Gaal's United in January 2015 and 2016, while only a last-gasp Zlatan Ibrahimovic penalty against Everton in April denied Koeman a hat-trick of Old Trafford victories.Nevertheless, the Toffees boss has been impressed with the side Jose Mourinho has built for this season and he is expecting a much tougher test this time around."It's how you look at it - maybe it's good that in the last three seasons I've had good results at Old Trafford," Koeman said."But Manchester United are really strong, a stronger team than last season and at the moment we're not at the level we were last season."