The summer transfer window has been a frustrating watch for Celtic fans so far. Not only have the club failed to complete any signings, but they do not even seem to be close either.

What makes it even tougher is the fact that the Rangers seem intent on strengthening again, despite how their heavy investment did not produce any results last season.

That shows ambition, something Celtic have seemed to lack for a while now. At least that is what it feels like from the outside looking in.

So far, the summer transfer window has been a story of one missed opportunity after another. And now, it looks like it is time to add the name of a Scottish centre-back to that list.

It is no secret that Celtic are looking to sign a centre-back this summer. They have already been linked with players like Zinedin Smajlovic and Oskar Wojcik for the past few weeks.

Luke Graham is another player who Celtic have seemed to have had on their radar for quite some time now.

Reports going back to February claimed that both Celtic and Rangers were involved in a race to sign the highly-rated defender from Dundee FC. He won't even be staying in Scotland at all though.

According to Scott Burns of the Daily Record, Graham has agreed terms with Stoke City on a 4-year deal. Dundee FC will get £2 million plus add-ons.

Surely the transfer fee could not have been a stumbling block for either Glasgow club. Burns does go on to state that the player's preference was a move to England.

Graham could have been a decent addition to this Celtic team. But would he have been a starter from day one? That feels unlikely. At best, he would have been a depth signing.

That is not what Celtic need right now though. They must bring in someone who is ready to come in and make a starting spot his own immediately.

There has been speculation that Celtic are one of the clubs 'bidding' to sign Nobel Mendy from Rayo Vallecano.

READ MORE: Transfer U-turn Gives Celtic Chance to Sign £6m Midfielder

Mendy had reportedly agreed to join Rangers last summer before a move fell apart in the final steps, although the reasons were never made completely clear.

He did go on to have a solid season of playing La Liga football for Rayo Vallecano. That has been enough to earn the interest of Celtic.

Clubs from the Bundesliga are also believed to be vying for Mendy's signature though. So, signing him won't be easy.

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